I rarely have a release word when I give commands. When I tell my dog "sit" "down" or "cross paws" I just expect them to do the action for at least 2 sec. If I want a long sit, I tell my dog "stay." In that case, the release word I personally use is "ok" but really I only use it for stays, waits, place, and "bang" (I want my dog to keep being dead until released).
There are a lot of trainers who never train a "stay" but just teach their dogs that "Sit" means butt stays on the ground until told otherwise. I think that is a great method, but I personally need to tell my dog "stay" for my benefit. There would be too many times where I can see myself telling Lance SIT and forgetting that I have to release, so I can see my dogs learning to self release.
For some tricks, I do work on duration, but I generally just teach them that if you continue to hold it you might get another treat. So with beg, I might give one treat 2 seconds in, and possibly another 10secs in. My dogs don't have to keep begging for longer than my usual 2sec rule, but choose to in hopes they may get another treat.