I have a //c that was purchased in 1985 that still works but it is rarely booted these days (not even once a year). I also would expect it to last basically forever because it doesn't have a hard disk. My floppy disks might demagnetize but I've got them all backed up so the data will never be lost.
Is the battery still on the motherboard? It will lose its charge or worse, corrode, and damage the motherboard.
I'm pretty sure it doesn't have a battery. No need since it doesn't have a clock.
My SE/30 monitor burnt out completely after a few years of heavy use. It was a sad day, seeing the old thing go.