Since time=money, I use this rule. I am currently rotating 7 chains on my 18" chainsaw while clearing 2.5 acres. I am cutting stumps about 1" below ground level. Needless to say, I am hitting a lot of hidden rocks and damaging chains. When the sharpened chain starts throwing "dust" (pieces of wood that are not thin, flat and look like like obvious shavings) within 3 minutes of cutting, then I throw it away. There is probably a precise scientific answer for this question involving wear on the chain measured precisely. I merely look at my right boot and when I see dust being thrown right after sharpening, it is time to junk the chain. A new chain will cut super fast until dulled by a rock, metal or lots of use. It can be resharpened about 5-8 times at a cost of $3-4 per sharpening. If you ever feel yourself pushing the chain bar down or trying to work the bar back and forth like a hacksaw, you have exceeded the chain usefulness. Put a new/resharpened chain on and keep going. You should NEVER have to force a chainsaw blade at all. Not up, down, sideways or any way. Forcing one is bad news. A dull blade needs to be sharpened. You are wasting your time trying to cut with a blade that is not throwing chips of wood that are flat, thin and look like shavings.
Make sure to have gloves or a rag ready. Bars and blades get HOT!!! I usually take a 2-3 minute break between chain changes. The bar will still be very hot after 3 minutes.