Time-based vesting implies full-time participation. In my case (and probably a lot of other cases), we are both part-time until the product gets traction. Then we will take the risk of going all-in.

If I tie the vesting to milestones instead of time, I should be able to fairly compensate the other person's part-time efforts without walking down the 'you aren't working enough, your shares aren't fully vested' path if/when we divorce.