Mowing can begin once the new lawn is 3" tall in most areas. Mowing stimulates growth and increases thickness. Set the mower high at first, then gradually lower. Try to cut no more than 1/3 of the grass at a time.
One general rule for mowing is that the shorter you want to mow the more often you need to mow to maintain healthy grass.
About one month after seeding, fertilize your new lawn using a high nitrogen (the first of the 3 numbers on the bag) fertilizer. We will install your lawn useing a high phosphorus or "starter fertilizer" when hydroseeding your lawn. The phosphorus will stimulate root growth, and in the early stages it is important to get good root growth before you stimulate a vigorous top growth with nitrogen.
We are always here to answer any of your questions. Give us a call and we can help!