There is no fixed timeline, and that uncertainty is part of what makes privilege revocation one of the more psychologically difficult punishments in the system.
The length of the revocation is entirely at the discretion of the facility, driven by two things: the severity of what the inmate did and how the inmate handled the aftermath. Contrite, cooperative, and quiet gets you out faster. Defensive, argumentative, or caught in additional violations while already on restriction makes it significantly worse.
For minor infractions, a couple of weeks without calls or contact visits is a typical range. For serious violations, that window stretches dramatically. A cellphone is one of the worst things an inmate can get caught with because it represents a direct challenge to the facility's security infrastructure. I did time with a guy at FCI Miami who had already been in the SHU for nine months when I arrived for a cellphone violation. He was still there when I left, and word was they eventually transferred him across the country to finish his sentence at a different facility entirely. That is the ugly end of what this can look like.
The facility being genuinely angry at someone changes the calculus in ways that no policy document fully captures. Staff have discretion, and an inmate who created a serious problem is going to feel that discretion applied against them for longer than the rulebook technically requires.
The most practical advice for the inmate in this situation is to keep their head completely down, make no waves, and give staff no additional reason to extend the restriction. Every day without an incident is a day closer to getting those privileges back.