ESPPs might allow you to contribute up to a maximum percentage of salary, like 10 to 15 percent. Some companies may also allow you to contribute a fixed amount. You can contribute at your chosen contribution rate until you reach the maximum allowable annual limit of $25,000 per year (for a qualified plan).
In either case, remember that contributions to a plan come from your paycheck — which means the more you contribute, the less take-home pay you have. Learn more