I think that as far as draft evaluation goes — at least for ~first round picks — three seasons is just about enough to know what you’ve got in certain players. Sometimes you get a player who improves immensely after three years; more often than that, you’ll see guys get considerably worse after three years, but for the most part teams are very aware of how they feel about their picks once those initial seasons are in the books.
Which is important, because as far as the 2016 draft class goes, franchises will have to decide on the fifth-year option for their first round picks once the 2018 season is complete.