Originally Posted by
Trueblood
^ - This. They wear together. Most of the time you have to replace the freewheel when you replace the chain. I have only had one of my vintage bikes not need a new freewheel after a new chain. That tells me the bike hadn't been ridden much before it came my way.
Well, it's true that they do wear together but you only need to replace both the freewheel and the chain if you let your chain wear too far beyond limits, which wears the cog teeth so that they are incompatible with a new chain. If you measure your chain regularly and change it whenever it exceeds 0.5% elongation (12 links measure >12-1/16") your freewheels will last a very long time. If you let things go much longer than that then you will ruin the freewheel (and possibly chainrings) so that a new chain will skip.