Quite a while, it seems.

They are all getting the completely new interiors with Polaris (only a handful left to go on the -300s then the -400s start.)

Meanwhile the -300s are the only airplanes in the fleet that fit in many existing UAL gates around the system. For example, they fit in the alley in ORD. This is a BIG issue until the new ORD terminal is finished in several years. Sure, they could use the exterior widebody gates, but then UAL loses a place to park an actual widebody. Repeat the same problem at other hubs.

Since the NMA appears dead, it would be interesting to see if Boeing would do a 767 MAX. The cargo carriers seem interested, FWIW.

Quick replacement with small 787 or A330s would present the same gate issues in the short term.

Since UAL is boxed in and the airplanes are fully depreciated it seems like UAL is happy to keep them flying until something changes.