MamaBB0304, as ADBONO stated, many roster decisions are made after Fall practice by the end of the Fall semester. However, the actual deadline to establish the 35-man roster is just prior to the first game of the season in February. Some teams will keep 36-38 guys practicing with them right up until the deadline and then cut a few at the last minute.
The difficult part for those guys that are cut at the last minute is that they don't then have the opportunity to transfer out to a JUCO or a lower level four-year program at mid-year because the spring semester will obviously have started and they'll be a few weeks into the Spring term.
Also, assuming those Division I programs follow the NCAA rules the way they're supposed to, players that are not kept on the 35-man roster are not permitted to practice or workout with the team once the season starts. As a result, those players will have to work out on their own in order to improve their skill level.