Don't worry about the length of time for starters; my OH has a hell of stamina and I'm normally down there for an least 45 minutes to an hour (though the time is getting shorter with pratice).
It takes time to work out what works and what doesn't, the best why to check, instead of asking all the time, is to read their facial expressions and body language. First start by doing something that you know gets him going and read his facial expression and body language...and then experiment a bit to see what works based on that reading.
Eg. My OH loves squeezing my ass while I go down and play with him, and I leanrt that when he stops squeezing he cant concerntrate cause he really likes what i'm doing and squeezes quickly in short bursts when he's cumming. Over time i've found other traits too.
Try and find something like that that works for you and it will be easier to work out what pleases your OH :) Hope that helps x