No ILB, you are not, I repeat not, to masturbate in your girlfriend's bedroom. I understand that, insofar as this week is concerned, her bedroom is effectively yours, as she is on study leave somewhere in the uninhabitable place known as The North, but nevertheless, it seems extremely churlish to take advantage of her family's generosity - that is to say, lending you her room for the week in exchange for your looking after said room combined with various bits of housework and generally staying out of everyone's way - by giving in to all biological urges and wanking furiously after a long and hard day at work. I mean, it's not even worth making the "long and hard" gag in this case.
Or, you know, any case.
I also don't care if you feel like you really need some sort of sexual release. Nothing's happened to make you feel this way, unless you count actually being in a room in which you have indulged in the deed itself more than a few times. It just happened - I am aware of this case, as well; it happens to everyone, you're not that much of a freak. But the thing that I am most aware of is that you have sufficient willpower to have remained a straightedge vegetarian for 16 years or so, and given the fact that you can do that combined with the fact that you actually know how long it has been, I have no doubt whatsoever that you can exercise said willpower to begrudge yourself the desired sexual indulgence, for the sake of gentlemanly dignity, at least.
(I'll let you know how this one turns out.)