More than percentages, it is the terms that matter. You can get a decent percentage of a early stage company, but end up making no money even if the company sells for 10-100x of the valuation at the time of stock options. Be sure you understand the terms.
Most of them are related to how and when your stocks are vested.
Most companies never IPO and get to a point where you can sell your stocks on secondary markets. So vesting period and terms of it are important.
For example - if you dont have accelerated vesting, you end up getting nothing when the company is acquired even at a good valuation. Depends on terms of acquisition as well and what happens to employee stock pool when acquisition happens.
