> I know it would vary from person to person, but can anyone tell me in general how fast you must gain weight to get stretch marks?

Good question. I'm not a physician, but I don't think the speed of your weight gain is the deciding factor for stretch marks. I believe it has to do with the elasticity of your skin and possibly the length of time your skin is stretched. Even with pregnancy weight-gain, (30+ lbs.), some women don't get stretch marks. Then again, some women get them once their stomachs reach a certain point.

On the other hand, I know of a person that gained less than 10lbs. and has dark stretch marks. So, it seems to me that it can't be generalized. The results from person to person are too different. It's almost like asking how an extra 20lbs. will be distributed on any given torso. Some people absorb it evenly all over, for others it might all settle in the abdomen, and others in their thighs...

On a positive note, I've heard that Vitamin E applied to taut skin might aid in it's elasticity.