How long until I’m fully bald/need to shave head?

I’m 31 now and first noticed a bit of receding hairline in my first year of uni (18). It has definitely come back a bit since then, but not crazy fast. Anyone gone through a similar balding pattern and can let me know how many years of hair I should expect to have before it becomes necessary to shave. Also, if there are any tips on retaining - all welcome. I don’t want to go on Fin, tried it but had side affects that aren’t worth it. Open to minoxil or whatever if it’s not crazy expensive. Thanks!