I agree with Dan and GB!! I use a blade until it stops feeling right. Just like you change out sandpaper on your sander when it stops working right, do the same with your blades. Some blades, however, don't last as long as others from the same batch. I've changed blades, and immediately wasn't comfortable with the way the new blade was cutting, so I changed it again. I think that's part of where the "artistry" lies, being able to feel and sense what's working and what isn't. You can't put numbers on the feeling part of cutting any more than you can put a time limit on how long it should take to complete a project. You cut at a speed and in a way that feels good to you. When it doesn't feel right, you need to make a change. Hope this helps!
Glen