In my experience, it varies. It's seldom planned in advance, and few who ask prior to an appointment if I offer it as part of my service find themselves able to take advantage of it.
Usually, the client just isn't "up to it." Or he's up to it, but just can't "get there" the second time, as much as he would like to and tries.
Some are very confident of their ability to fire a second shot and find themselves admitting, much to their dismay, that they're just too worn out after the first one. Then there's the occasional one who was surprised to find that, though he is seldom able to "go a second round" with anyone, for some reason, he felt "inspired" enough with me for it to happen.
I'm a mature escort, most of my clients are over 45, and it's not really important to them—they seem to put quality before quantity. In fact, many have medical concerns like diabetes, which causes them anxiety over getting off even once, so MSOG doesn't even cross their mind.
As for the number of my clientele that actually achieve MSOG, I would say it's about 10-15 per cent.