I'm not aware of a time limit to how long the Symbiote can live without bonding with a human, what does it gain from doing so?
TL;DR: The Symbiotes used to absolutely need a host because they were adrenaline junkies. Nowadays, they don't need a host (whether it be human or something else) for survival, but they need it to reach their species' purpose. Having a host helps:
- To help people out
- For communication
- For food
- For food of the mind and self-definition
- For self-control
- For vesseling and environment awareness
Previous version: Galaxy's bane
Back in the day, the Symbiotes used to be addicted to the emotions of the host creatures, as they did not have feelings of their own. The result would be Symbiotes wiping out whole species as they pushed the hosts to lethal exhaustion. Thus, hosts had a "limited term of utility" (panel is mildly NSFW: corpses).
The Venom Symbiote was an aberration among its race, as he explains to Eddie in Venom Super Special (1995).
In Venom: Along came a Spider #1 (1996), we meet the Hybrid Symbiote, which is formed by four Symbiotes which were being experimented on. This Symbiote escapes and seeks a host (Scott Washington); the terms it uses imply there is indeed a need, without much explanation given, though.
Starving, alone, together but alone; need Other, kind one, Scott, need Scott and Scott needs us...
However, this same Symbiote is featured in Venom: The Hunted (later in the year 1996). At this point, Scott states that the Hybrid Symbiote needed a host "to survive". Since the wording implies said host had to be human, it's possible however than this very condition is only applicable to Hybrid (maybe due to the fact it used to be experimented on).
Current version: Galaxy's saviours
Nowadays' Symbiotes, however, are not the mindless junkie conquerors they used to be presented as - or at least, that is not how they behave. De facto, having a host helps...
To help other people out
Yes, the Klyntar may be a species of monstruous-looking black goo aliens seemingly parasiting bodies, but apart from the appearances and the dozens of Symbiotes which eventually turned into rogue psychopaths, the Symbiotes want to bring peace and fight for the greater good.
Problem being, it's not exactly easy to do when you're a black goo thingy, so while they are "the mind and soul of the warrior", they also need "a form to act out what they know to be the true ways of a noble and virtuous galaxy".
In Guardians of the Galaxy #23 (2015), the Guardians reach the planet of the Symbiotes, and this is pretty much what they tell them, while also explaining how the numerous rogue Symbiotes can be explained by the delicate balance needed between the host and the Symbiote to reach the aforementioned goal.
When your goal is to travel the galaxy and help other people out, you might want to communicate a bit. Especially when your Symbiote form looks like a drugs-induced nightmare with several teeth rows: being able to speak out your noble intentions helps people seeing you as a friend, not a foe.
Then again, verbal communication can only be achieved by bonding to a host, as shown in the same issue, where they need the Guardians to bond with them.
Symbiotes thrive on phenethylamine, a chemical found in chocolate and brains. Some Symbiotes don't mind consuming people, too. Access to these things is made easier when you live in a fauna, or a big city with people and supermarkets; I have yet to find any mention of a Walmart in their home planet.
For more details on what Symbiotes feed on, I shall redirect you to What does the Venom symbiote subsist on? (disclaimer: one of the answers there is mine)
Food of the mind, and self-definition
Klyntar planet seriously lacks art, music, literature, in a word, culture. All the Symbiotes have is their hosts, and as such, the relation between a Klyntar and its host is sacred because it shapes their history; even more so when you know that Symbiotes retain memories of their previous hosts. That defines them.
Panel below is taken from Venom #3 (2017).
For all their powers, Klyntar also have their fair share of weaknesses, the main ones being fire and some sonic frequencies. When faced with one of these, they're not good at self-control. Think freaked-out dog. They will not think straight, they might attack those around, and hurt innocents - which is the very thing they try not to do. As such, the host is there to keep things in charge.
Panel below is also from Venom #3 (2017).
For vesseling and environment awareness
Knull, who is basically a deity and the origin of all Symbiotes, describe his origin in Venom #4 (2018) (nb: that's not the same series as above). After being beaten up and stranded on a desert-like planet, he discovered he could bond with the "lesser creatures" that were there, and that such symbiosis could be of use to gather information around. This was the first instance of symbiosis.
Worth to notice anyway, that most of the things above apply for "good" Symbiotes. Rogue ones (some incarnations of Carnage come to mind) tend to be a bit more on the "just want to kill and it's easier with a host" vibe. Then again, one does not simply try to understand why those would do the things they do; sometimes you need a monstruous-looking alien to do what's expected of it, meaning go around and mindlessly kill people.
When the Venom symbiote was healed by his people, we got the following explanation of why the Klyntar seek to bond with others:
So, apparently, Klyntar bond to hosts not purely for survival, but to become cosmic do-gooders (or, as stated in the Venom: Space Knight series that followed this event, "Agents of the Cosmos".
Or, for shorter term reasons like communication (as happened immediately before the scene above; the Guardians allowed Klyntar to bond to them, because that was the only way the Klyntar could fill them in).
We saw some support of this idea (that the Klyntar as a rule don't require a bond for survival) in the recent Venomverse / Poison-X / Venomized trilogy; the Poisons (some sort of extradimensional race that can take over other bodies if the others are bonded to a symbiote, killing the symbiote and the other's mind in the process) kidnapped basically the entire Klyntar race from their homeworld, performed some sort of genetic modification to make it practically impossible for a human and to separate once joined, and forced them to bond to humans with powers. At the end, the KLyntar (as a race) were perfectly happy to be separated from their forced bondings, and go back to their homeworld (as seen at the end of Venomized #5).
So, to answer your question - a symbiote doesn't need to bond to a host; normally, they can survive indefinitely without bonding to anyone.