When is it too late to declare a scholarship player to red shirt?
Scholarship or walk on, does not impact red shirting.
Per NCAA rules, there are two types of red shirt situations - 1) Traditional, 2) Medical.
1) Traditional Red Shirt (my term - the NCAA must have a better sounding term than mine) means that the player was perfectly healthy but did not compete in ANY matches, not even one point. In football this type of red shirt is quite common and allows incoming freshman the time to learn a certain offensive/defensive system while getting physically stronger. For Volleyball, this tends to happen for freshman who have seniors entrenched in their positions (most often Setters/Liberos) or if a very athletic player does not have club level experience and needs time to catch up with their skill development.
2) Medical Red Shirt - This is the result of an injury. The NCAA uses a formula to determine a SA's eligibility to obtain a red shirt if they sustain a season ending injury (I believe the school's training staff have to also fill out paperwork supporting the validity of the season ending injury). It is my understanding that the formula says a SA can only participate in a maximum of 30% of the team's dates and no competition after the halfway point of the season. This generally works out to around 6 to 8 dates and none can be after the midway point of the season. All of these nitty gritty details can be searched at the ncaa.org website under Legislation/Rules.
The tough thing is when a player gets seriously hurt in the first half of the season, but has played above the maximum allowed per the formula.
The NCAA does also allow for the awarding of a 6th year, in those unfortunate circumstances where an athlete suffers two separate serious injuries and has already used their red shirt year on the first injury.
Hope the answer helps and I encourage you to search through the ncaa.org website to confirm the exact formula for determining red shirt eligibility with an injury.