Most incarnations of The Hulk I've seen or read about state that he is as strong as he needs, basically meaning that there is no limit to how strong he can get.
Is there any canon regarding this? Have we seen the upper limit of the Hulk's strength?
As of World War Hulk, Hulk has been able to withstand and beat the strongest Marvel character at the time, Sentry (Void/Angel of Death). Hulk was not defeated by Sentry, instead he surrendered and turned back into Bruce Banner. To compare, Void almost destroyed all active superheroes in Siege, again only being killed when he turned back into human form and begged Thor to kill him.
As for upper limits of the Hulk's strength, as with all comics, it depends on the writer and the plot. At any given time though, Hulk is always considered a threat on par with a force of nature/act of god, unstoppable total destruction.
From the Marvel wiki:
Class 100+, although this strength chart is strictly made for comparison purposes between characters, and it is not a literal description. The Incredible Hulk is one of the physically strongest beings in the Marvel Universe, and the strongest mortal to ever walk the planet Earth. In human form Bruce Banner possesses the normal human strength of a man of his age, height, and build who engages in no regular exercise. When the Hulk gets enraged his physical strength increases exponentially. The Hulk has no strength limit, his strength varies up to literally unlimited levels, according to the omnipotent cosmic entity Beyonder. Most recently the Mad Thinker's mathematical computations also inferred that the Hulk's strength is indeed incalculable. Additionally, as the foundation of his phenomenal strength, his rage is also described as being limitless.
I've read every Hulk comic (including guest appearances in other mags) from his origin in '62 through '76. That's 14 years down, 40 to go.
As is stated in nearly every one of these comics, Hulk is the strongest one there is!
This is basically the definition of the Hulk. No matter who he is up against, he's stronger. Take the strongest possible adversary, invent someone who is stronger, and Hulk is automatically stronger than they are.
Hulk has fought men, monsters, gods, dreams, and even psychological constructs. No matter who the opponent, he comes out on top due to sheer physical strength.
Even if an enemy starts the fight stronger than Hulk, there is another line found in many Hulk comics: "the madder Hulk gets, the stronger he gets!" So, by the end of the fight, Hulk always gets mad enough to be stronger than his adversary.
As an example of this theme, I picked a random comic, The Incredible Hulk Vol 1 #194, and checked it and the next four comics. Here are the results:
It was stated by the beyonder that the hulk has no limit to his strength nor power. According to this answer on Quora:
there is in one issue where the beyonder said there is no limit to his strength and I think he was talking about his cells
There's no upper limit to his strength and falling to his knees after having the weight of a star dropped on him is probably his greatest strength feet (he was in the process of getting back up, Infinity #6, 2013). He can easily lift a planet, let alone a mountain. He shattered an asteroid twice the size of the Earth with a single blow (Marvel Comics Presents 52, 1988).
When addressing such questions, one should avoid comments made by other characters (except perhaps, TOAA, or the Presence). Such comments are hearsay. Thus we should only give credence to empirical evidence or primary sources such as the authors of the "Supreme Being" of the comic company. An example of empirical evidence might the the 52 version of Superman bench pressing 13,170,000,000,000,000,000,000,000 pounds constantly for 5 days.
Think about it!! How could there possibly be empirical evidence that Hulk could have unlimited strength. I’ve seen no primary source which has said more than Hulk’s strength increases with his level of anger. That does not imply that his strength is without bound (for one thing, his level of anger would also be limitless). Here is an example of increasing strength with anger levels. Each number represents a level of his strength based on increasing levels of anger: 10, 10.9, 10.99, 10.999, 10.9999 … Clearly his strength is always increasing but is not unlimited.