I used the massager for a duration of about 20 minutes right after laying down for the first OO/GF mix. I got a little nauseous from it - ume paste to the rescue - so I decided to not push it too much.
But right after the next OO/GF mix, i massaged for quite a while... I was watching a movie, so pretty much on and off during the whole movie.
I'd just trust your instinct and see how it feels.