What is the climate where you are? I will assume you are talking about lawns rather than the grasses associated with prairies, pastures, orchards, meadows, etc. There are warm season and cool season grasses and for each of those several species and numerous varieties to chose from. Some grasses will germinate in a week others will take much longer up to 3 weeks depending on climate and season. Preferred seeding of grasses in the Midwest (a temperate zone) is in late summer/early fall, when soil is warm, plenty of moisture and air temperature is cool. Fine bladed tall fescue mixes are commonly used in the Midwest because they are tuff, deep rooted and draught resistant. It is best to seed tall fescue at a rate of 10lbs/1000 square foot. You will hear of 6lbs and 8 lbs per 1000 sf but it is not recommended if you are looking for a dense lawn with no feel of the individual grass plants under foot. All lawns take a full growing season or more to mature.