Jeff there are several things to be aware of to determine the need to change the blade to a new one.
1) smoke coming from the cut and or the smell of burnt wood
2) you are having to apply more pressure to get the blade to cut.
3) the project starts to chatter when you are cutting and it did not do that before
4) The wood you are cutting is hard such as - Brazilian Maple, Bamboo, Yellow Heart, Purple Heart you will need to change your blade more often.
5) You are having trouble controlling the cut.
Basically if you fee that the blade is getting dull then change it.
DW