No, the grantable levels do not disappear until the 90 day RaF period ends (or they are used). Your level 90 will be able to grant 45 levels. Characters less than level 90 can only grant a character to their current level. So if you use a character(s) with enough grantable levels to get a boosted character to level 30, you will need a character with at least 1 level who is level 31 or higher to boost that level 30 character to level 31.

- Souca is Doing Too Many RaFs Right Now