The San Francisco 49ers will cut down to their final roster in just over three weeks. When the 53-man roster is set at that point, it will likely not be completely finalized. We will see roster moves in the days leading up to Week 1, and there could be a flurry of roster moves immediately after Week 1 wraps.
The reason for that latter prediction is due to NFL rules regarding veteran contracts. Generally speaking, NFL contracts are viewed as not guaranteed, barring some additional negotiation between player and team. However, once the regular season starts, certain salaries become fully guaranteed for that season.