The images below are blades that I have in my shop. They are Flying Dutchman blades. These are generally considered high quality blades. You can especially see the damage caused from cutting in the 200x magnifications. I change blades very early in their life. They are cheap and I don't see any reason to ruin a project for a 25 cent blade. If you use a high quality blade they will last much longer. I usually am able to get through a typical project with 1 or 2 blades. By typical I mean the average pattern in my catalog.
20X magnification of a #3 FD Ultra Scroll Reverse.
You need to learn to recognize the symptoms of a dull blade. You will start to notice that you have to push the blade through the wood with more force. Sharp turns will become more difficult to make. Sometimes the blade will not track the pattern line as accurately. In some case you will begin to see and smell burning of the wood.
New Blade 200X magnification.
If you start to notice any of these symptoms, replace the blade. If the problems disappear then you can log those symptoms in your memory as a dull blade. Over time you will recognize the symptoms more quickly and your cutting will benefit from that experience.
Used Blade 200X Magnification.
Sometimes it's okay to push a blade longer. I will sometimes push a blade past it's life. If I'm cutting a soft wood and the pattern does not have intricate fretwork I will just push on. I still have my limits though. If I get any burning the blade has to go. If you are experienced enough you can adjust your cutting to compensate for a slightly dull blade.
If there is intricate fretwork or the wood is tough to cut then I don't even try to push a blade. Like I said earlier, blade are cheap compared to my time and the wood.