Air conditioning window unit

Sounds like your compressor is dying. You are probably on borrowed time. Hard to say how long it will last, because it depends on how much the unit is running (depends on climate).

A total lifespan of 15 years is reasonable for a unit before major repairs are required.

Note that newer systems are usually far more efficient (SEER and EER ratings). And so they can pay for themselves pretty quickly. Especially with the tax credits that are available now.

We resolved the problem with the portable AC by purchasing a heavy-duty, 15-amp, 12-gauge extension cord and running it from the bathroom to the AC by the window. Since the bathroom is on its own circuit, the AC does not trip the breaker. A big thank you to those who suggested this in the comments.

Ironically, management has decided to replace our central air conditioner after I discovered mold in the unit, and their subsequent inspection also revealed it to have a serious Freon leak.