[xg]

Standard is a 4 year vesting period. So, for example, if a cofounder had 100,000 shares, they would vest 25k shares each year--possibly with a one year cliff.

If you've already put considerable work into a company prior to any financing event, etc, it's common to keep 50% of your equity upfront and vest into the remaining 50% over a four year period.

Continuous periodic vesting is important. It's usually set at a quarterly or less time interval for the reasons mentioned by markstansbury.