The risk of investing time in a startup is in the lost opportunities (ie. opportunity cost). It can be dollarized with discounted cash flow as an approximation. Could you earn more at a large company than at a startup? What are your options? Do you even have better options? How high and how likely must the "lottery ticket payout" be to break even? And don't forget the time value of money: How much could you gain by investing the excess money during those years?

Time is the most precious resource we have. It's per-se underrated whenever it's dollarized. Unfortunately, most don't have a choice than to sell their time for money. Thus, it's even more important to spend time wisely.

Thinking, you don't risk anything because you don't spend money on a startup is wishful thinking. Or more likely: It's just a good story VCs and founders like to tell, because it's in their interest.