> 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?

Any science teacher will tell you there are many variables that can change the outcome of your experiment.

Stretch marks depend on your age, weight, size, gender, type of skin, speed of weight gain or loss, general health, physical condition, eye color, the climate where you live, and lots of other stuff I haven't thought of yet. I think made up the part about eye color, but who knows?

When I was 22 I gained 30 pounds in just a few months. No stretch marks. When I was 69, I lost 30 pounds, and again, no stretch marks. The birth of our childern didn't seem to give me stretch marks either, but another member of the family got some. She still has them, but we think of them as a badge of honor.
Howard