when to pull the plug??

And/or pump. (Compressor part)
If it's electric then it's probably the compressor part that is wearing out.

Even though it's electric the wear on the compressor section is almost like our motors in our cars and they wear in almost exactly the same way.
Rings, Pistons, valves and seals are of different types and materials but perform the same basic funtions.

See if you can find a rebuild kit from Sears and go for it.