In the earlier seasons of DS9 it was established that Odo needs to spend at least some time in his liquid form every 16 hours (which he referred to as 'regenerating').
Does this apply to the rest of his species, and if so, how do the ones posing as humanoids in other cultures avoid showing up on ship/station/planet-side sensors? (Two questions I know, but they are connected.)
Unfortunately, the need (or lack of need) for regeneration by Changelings other than Odo is never addressed directly in DS9 episodes.
However, we can reasonably conjecture any one of the following things:
If they have a hard limit of holding their form for at most 16 hours per day, that still gives them only 8 hours each day where they must "sleep" — which is convenient. A changeling imposter can simply retire to his or her quarters and pretend to "sleep" while actually regenerating. (During extenuating circumstances when there is no designated sleep period, a Changeling imposter may be able to schedule short bursts of regeneration during private moments instead of a continuous period of regeneration.)
Some Changelings are far more skilled at shapeshifting than Odo — and successful infiltration requires great skill, as they will need to mimic humanoids exactly. These skilled Changelings may be able to hold their shape for longer — possibly days on end without having to revert to a liquid state.
Changelings spend most of their lives in liquid form in the Great Link. It could be that this regenerative time accumulates. Perhaps the longer one spends in liquid form in the Link, the longer he or she can spend in solid form.
Changelings, through the Vorta, have access to extremely advanced medical knowledge and technology. (In Season 7 of DS9, we see efforts by Vorta doctors to cure the Founders.) There could be medical means of shortening a Changeling's regenerative period.
Again, this is all pure speculation, but compatible with what we see in DS9.
Curious enough, on the recent Picard episode No Win Scenario, a changeling posing as an officer on the USS Titan did seem to need to regenerate (a container he used as a receptacle was found hidden in the quarters of the officer he replaced) and in the previous episode Seventeen Seconds, a suspect Worf and Raffi were interrogating revealed himself to be a changeling when, after staying in humanoid form too long, he involuntarily reverted to his liquid state while being questioned.
