What is the life expectancy of a Powerbook G4 hard drive?

Even new hard drives can fail- often without warning.
Be sure you are fully backed up at all times.

On my Pismo, I got seven years out of original HD. The two subsiquent replacments (new) I’ve been lucky to get a year & a half out of each!
They just don’t make em like the old days;)
Run “Disk Utility” and repair permissions.
Once finished, highlight HD and see what it says on the very bottom re “S.M.A.R.T Status”. If it says verified you should ‘generally’ be ok. But if it’s making noises… I’d make regular backup and pray to HD gods a lot :o
—a couple of sacrificial mice might not hurt either.—

Indeed, every hard drive will fail eventually, which is why backups are so important, as mentioned.

There is really no guideline or average life on hard drives. Some will last a year, some will last 10 years.