The actual process can be a day or two but rotating the aircraft into a facility capable of doing the mod may take some time due to logistics on all sides (e.g. can the airplane be spared from the operations? Does the MRO facility have a hole in their schedule that this can be fit into?) so it may be weeks to months before its done.

Another wrinkle is with UA rolling out Starlink for the wifi solution in the near-ish future, at some point UA will decide not to fit any additional aircraft with the legacy equipment to avoid writing off too much of that investment which means some planes will be flying dark for longer than usual -- I'm not sure if UA is to that point yet, but IIRC The UAX fleet will be coming first.