Hey Jim, As a scroll saw instructor at Woodcraft in Ventura, Ca, I tell my students when the blade no longer does what you want it to do, It's time to change it. It could last for an hour of cutting, or it might last one minute. there have been times when I have put a new blade in and it wont cut like I want it to. It gets pulled out and chucked.
When you have to push the wood and the blade starts deflecting, it's time to change it.
Let the blade do the cutting. Make sure your blade tension is tight. You want to hear a high ping when you pluck it, and use either blue painters tape on top of the wood, or clear packing tape on top off the pattern to help cut the friction of the blade. The blade will cut easier and last longer.