You Should Consider Deadlines and Act Promptly
On the same day that you receive the proposed severance agreement document you should check it for how much time you have to sign and return the agreement. As mentioned above, usually an employee will have 21 days or 45 days to sign and return it, but it can also be 1 week, 10 days, or another period of time.
You should also look over the contents of the proposed agreement promptly, and if you decide that you want to have an attorney review it/potentially negotiate more and better severance terms, then preferably within a day or so you should make arrangements to meet with an attorney - the more time the attorney has to review and negotiate back & forth with the Employer/ the Employer’s attorney, the better off you will be. In this same regard, you should not wait to the last day (or later) to act: waiting to the end will just serve to put your back against the wall (although sometimes in that situation you/ your attorney can get contact the Employer and more time to consider/ negotiate the agreement).
To put yourself in the best position to negotiate better and more severance (generally by and through an attorney that you will hire) you should act promptly and not delay.