As others have said, you set boundaries to suit your own comfort level but if I get anyone wanting prolonged oral, I mix it up with long slow licking, tongue flicking over the tip, playfully rubbing the tip between two fingers etc i.e. anything apart from sucking hard for sustained periods. I'm strictly no face fucking for many reasons but not least due to comfort and control.
If they ask to book a session of just or mainly oral, I make it clear that I regard it as edging and this gives me leeway to ease off every so often if I start to ache.
I also make sure I'm lying or sitting in a position that's comfortable for me and I always keep a glass of water next to the bed to keep the saliva flowing if I'm getting thirsty.
I have one regular client who always has an hour of edging via oral, we get settled into position with pillows etc and off we go. Last time I noticed his right leg stayed in the exact same position for the entire 55 minutes we were on the bed, god knows how he didn't get cramp.