Maybe when its not a nice clean cut or you get burning as you push the wood through??
If you know the saw well, youll be able to feel if it needs changing.
Someone always used to change the blades for us, If it didnt feel ok when pushing the wood through we'd get it changed, Now im paying i would probably leave it a bit longer. Just changed my RAS blade after 6 months use, cost £60+ for a good swedex blade but well worth it.
Something i learnt, if buying a new blade, My old boss bought a thinner Freud Kerf blade than the old one, Always wondered why it was hard to push wood through, seems it was thinner than the riving knife so would give resistance when it touched it. I didnt know about all that and abviously my Boss didnt either.