You can begin mowing after three to four weeks, or more than 4" of new growth. Mowing will help stimulate root development and cause your lawn to thicken even more. Avoid heavy riding mowers at this stage. Be sure to bag all grass clippings for the first three mowings.
If you have weeds of any kind, please do not use any weed-killing product on your lawn until it has been mowed three times, as the poison may kill your new lawn as well.
Your newly hydroseeded lawn contains all of the fertilizer needed for three whole months. After that time, your lawn will greatly benefit from regular feeding. Be sure at the end of the growing season to apply a winterizing fertilizer. This will make a big difference for your new lawn next spring.