Curious minds want to know.
No formula. A function of how often it's used and how tight it initially fit. e.g. If super tight (i.e. you need helpers to put it on and can hardly stand up straight when fully zipped up) will stretch out faster than one that fit "loose".
Typically my understanding is that suits are sized to be really tight initially and will conform to the racers body as they wear it a few times. I could be mistaken, but I believe the DH racers might only wear a suit 2-3x before getting a new one (I've actually heard of some getting a new suit every race as they want it super snug and the material to have the maximum slickness)
Curious as to the question why? Are you wondering for your own racing?
Unless one is WC level one isn't going to notice any performance differences between a new suit and "stretched" one provided the suit is still tight and doesn't have any baggy spots. In short course masters racing, I'd say it's more racer skill vs. suit or no suit. In longer courses suit is beneficial and the differences between a perfectly fit, super snug suit and one that is tight but might not be perfect everywhere is pretty inconsequential.
In Nastar courses I've run when I was racing for a weekly Night League team, the suit will take off a full second.
Others racing the same course tended to agree.
Better technique of course will take off 3-5 seconds on those courses, but it is harder to buy on eBay than a spandex suit.
I bought a really nice Spyder 99 GS suit. It was so hard to get into I did not make it the first time. Tried again and was able to get into it but couldn't get it off. Finally figured how but not before overheating and going to the neighbors for help. Glad they were not home as it would have been really funny for a green frog to show up at your door. The suit has a really nice layer inside, great padding but truth be told I doubt I need it. At 69, returning to club racing after a 20 year hiatus it is going to be while before i have the competitive numbers to j justify a suit. . On the other hand I paid $139 for a $435 suit and I hate to give it up. I wear a L and this is a XL and that is why I was asking about the suit stretching. I really like the padding. Hate to give it up but I should.
Yes they do, and that is why top WC speed skiers generally only wear suits once to race in. As they stretch, they become slightly more porous and that makes them fractions of a second slower. Might make a difference to someone at that level, beer league racers not so much.
Karl, you are correct, same here - but after about a year, there is that under hang on my rear end that I constantly tug at - middle is expanding, behind getting flatter.
I recommend a particular seamstress/tailor than can make just about "anything" to work including tricky cross-gender applications.
His name is Bill ... I think he goes by the nickname "Buffalo Bill". Join him in his shop for a expert fitting and a fun time.