it really depends on you
if you are rough on arrows they wont last as long as if you go easy
what I mean by that is what type of medium you shooting them into
depends a lot on how much you shoot if you shoot every day 6 months

once a week a few years

pull the bow out the week before season and goes back in the closet the day gun day opens untilnext Bowseason, you may never have to replace them

Bottom line is how often you shoot.

you can make a spine tester pretty easy that way you can chech them new and record the spine deflect and check them periodicaly.

I think Petes place has plans on how to make one