The earlier you signup the more seniority you'll have within your travel category so your goal should be to have as much seniority (days) on the Space-A register without having your signup expire when you are competing for a flight. All passengers "should" signup at every location where they might need to compete for seats which means Active Duty "should" signup at potential departure and return locations as soon as their leave status begins. With the exception of active duty, signups are only valid for 60 days (active duty is the duration of their leave).
With that in mind, many retirees and unaccompanied dependents plan their signup date so that they have approx 50-55 days on a location's Space-A Register on the first day that they plan to attempt travel. That allows them to have subsequent Roll Call opportunities before their signup expires and they need to re-signup (revert to the bottom of their category).