Question: I only use my table saw and circular saw, both with ripping blades, a few times a year. How can I tell when it's time to replace them?

I imagine I'm not going to lose them to wear, but then again, maybe I've hit an occasional nail. I don't know whether storage and rust cause degradation or if any rust is superficial and gets knocked off when I use them again.

Answer: 
Your blade will start to show signs of dullness. These include:

* Binding
* Excessive force required to advance cut
* Excessive tearing/chipout
* Burning the wood

It helps if you can remember the "feel" of cutting with a sharp blade to help recognize these signs. One of them alone may be a result of improper technique. But when multiple signs stack up, you can be more sure that it's time to switch out blades.
Sometimes when people believe their saw is dull, what it really needs is a cleaning. The resins from the wood build up in the teeth over time and gum up the cutting edge. Soaking the blade in simple green or another cleaner of choice and then scrubbing the teeth with an old toothbrush can prolong cutting life.