Prickly, we're not all seeing a zillion guys a day and as each encounter is different, we don't always get oral with each booking. Personally I find the stimulation increases a little each time, so the next person doesn't have to put quite so much effort in to get the same result and if someone makes me gush, then the next person may achieve it a lot easier than the first, as my body is more switched on to that. Sex is a bit like a drug, as in the more you have it, the more you want it.
Personally if I ever get to the stage where my body is tired or over stimulated, then I stop taking bookings for that day and then I'll be ready for the next day, as bodies are wonderful and with a little rest are all ready to go again.
Having said all that. If you have one client who has sharp nails or is a little rough, it can put you out for days, so it is really important to know what you're doing, as we're playing with delicate areas.
I can't imagine a man ever getting sick of blow jobs.