I am pretty sure even after you graduate and get an OPT, you are still on F1 visa. But the point is 29 months (if you are a STEM major) is enough for your startup to generate some revenue, and then you can apply for H1b (as long as you prove your startup has sufficient fund, your wage from your startup is prevailing wage, and you can be fired from your startup).

If a startup fails, it usually fails quickly. Yes, sometimes a company, like airbnb, went quiet for a very long time before it hit it off, but 29 months are likely enough.