You can't. That is what distinguishes you from a VC. Actually, you can. Serially. Plenty of people work for a startup through its early stage. A year here, a year there. That's diversification.

But how do you determine when it's time to jump ship? A year seems awfully short to make that determination.

If your object is diversification of your investment then one year isn't too short. However, typically your option vests monthly after the cliff. So you don't have to stay for two years unless you want to.

Bounce around until you find somewhere that has a rocketship trajectory? If you can work at 10 startups over a 20 year career, your odds aren't terrible