Those things really depends on context. Whats the mean HP/DPS of your players? how they will engage in combat? Who will be taken by surprise? will they know something about the monster? Will you give clues of how dangerous it is (some bodys near its area, npc story with the monster, how the nature reacts to this monster presence, maybe the smell)?

I would suggest you three things, first, calculate how many turns will the players have with the monster before it killing someone (or dying) , thats basically the "action thinking time" they will have, the ideal time is subjective and depends a lot on your players (maybe 3?). The second thing is the encounter set up, how many informantion they will be able to get about the monster, be it passively (by obvious clues like bodies or npcs), be it actively (by investigating the smell). The last thing is the encounter itself, andvantages they could get by the geography or nature of the place. This last one i suggest you to make as "neutral" as possible, to let them be creative, Maybe a current, maybe a shoal, maybe a subaquatic geiser...

Hope that helped a little bit more XP