What I do at my theatre . . . which specializes in classical theatre.
We do four days of rehearsals, with the understudies in the roles they are understudying. (If an understudy understudies two roles in one scene, they we work the scene twice.) I DO NOT call the regular EQUITY cast members to stand in, but I usually call the NON-EQUITY normal cast to help fill in the gaps as needed.
During these four days, we also do the understudy costume fittings, understudy vocal work, understudy fight and dance work. I work very closely with the AD, who actually is directing the understudies.
On the fifth day, we finish all the fight and dance work, and then do a little bit of tech. (Like speciality items). The last thing we do is a understudy run thru, with the understudies playing the role, we add in lights and sound (no costumess), and the rest of tech.