As for hunting itself, basically there's B ranks, A ranks and S ranks. 'B' ranks are only worth killing if you have the weekly hunt bill, they don't give anything if you don't have the bill. 'A' ranks spawn every 3-5 hours, and 'S' ranks window opens around 60-80ish hours after previous one killed.

A ranks don't spawn at a set time. Basically, after a particular A rank is killed, a set time will go by and then their window will open up again with a chance to spawn. If after 1 hour (or 2 hours for Heavensward A ranks) they haven't spawned, it will force spawn.

S ranks have a longer respawn time. Once their window opens, players have to try and 'trigger' the S rank. Each S rank hunt mark has a different trigger, and doing the trigger has a chance to spawn it.