It’s hard to put a timeline on someone’s comfort level, but the general rule of thumb should be this: If she holds out on sleeping with you for a month, that’s reasonable (assuming you see her three or four times during that window). Two months is pushing the boundaries of patience a bit, and at three months – or ten dates, whichever comes first – if her tower still hasn’t cleared you for takeoff you’ve got a problem.
Of course there are exceptions and nuances to account for in this equation. If geographic boundaries impede you, you can’t apply this thinking to a long distance relationship. One could argue that, not being afforded the opportunity to see each other often suggests that she’d want to maximize those visits, but you can’t really expect her to give it up if she’s only seen you twice in several weeks.
Since we know immediately upon spending time with you whether we’d ever let you touch us or not, if your lady has you on lock down, something’s amiss. Maybe your reputation precedes you and she’s weary of your track record enough to make you earn it. Nothing worth having is easy to get, after all. Or perhaps she has body image issues to work through. Or it could be something much weightier than her weight troubling her (read: past trauma), in which case handle her with velvet gloves. When she’s ready to tell you, she will.
The only other obvious possibilities are that she’s either not a very sexual person (in which case you have my permission to run) or she’s not terribly attracted to you – hard to hear, but true.
Whatever her reasons, she should understand that patience is a virtue but men are sexually-driven creatures who will only hold out so long before forfeiting the game and moving on. Be respectful if you think she’s worth the effort, and remember: a gentleman always avoids unsportsmanlike conduct.