Details depend on the aircraft and your SOP.
While you cannot legally descend on the glideslope until you are on the localizer, some older automated systems (e.g., 747 Classic) will do it if set up. Newer systems (e.g., 744) will not capture the G/S until the LOC is captured.
An old rule of thumb is the "minute to live" rule -- max 2000 fpm descent below 2000' AGL and max 1000 fpm below 1000' AGL. At high-altitude airports near max landing weight, you may need 2000 fpm or more to get down to the G/S, and may be at 1000 fpm or slightly more when on the G/S.