How long should I tickle my girlfriend for?

So, it's my birthday next week and my girlfriend has agreed to let me tie her up for a tickle session. I have a bit of a , I'll admit. She is very ticklish but I usually restrain myself, except for a few pokes every now and then. But now she's agreed to let me tickle her for as long as I want. I can choose the time to stop but I wonder what's fair.