If 99% of the blade is sharp, but the 1% that isn't sharp is in the material tearing instead of cutting . . . then the blade isn't sharp "enough".
I still use #11's, and I've gotten them in 100 packs that work out to ~$0.20/blade, but when cutting out most models, I typically average swapping a blade every other board, but that isn't as much from wear rate as much as why it's no longer "sharp enough"
Most common blade damage until you know to look out for it? The broken tip. particularly if you're being bold and trying to cut all the way though on the first pass, it's VERY easy to make a sharp curve and feel a very subtle "ping". pull out the blade and you no longer have a point. it's now embedded in the cutting mat. because of the twisting break, the end now has a weird off-axis edge on the bottom that will drag and tear while the rest of the blade is cutting, so the next thing you cut will turn out horrid, even though the edge appears keen. How do you avoid it? shallow cuts, particularly on curves. I tend to take two passes for straight cuts -- first pass cuts the inside paper and a little more than halfway through the foam, then a second pass to finish the foam and *mostly* cut the outside paper. the idea being avoiding cutting the cutting mat any more than necessary. on curves, I hold the blade nearly perpendicular to the board (easier to turn around the radius) and make 3 cuts to make up for less blade in the material, with the final cut is generally done while lifted off the cutting board.
Second most common? For me, Adhesives. Trim off a piece of tape? Cut a hot glue seam? Cut through a tape-reenforced seam? usually something sticks to the blade as it cuts through. Usually, when the builds starts to getting to the clean-up stage is when I'll have to make a blade change, because the blade is now dragging from the gunk that's coating it's surface. In many cases it can't be avoided, but if you just swapped in a new blade, before you drop that old blade into the sharps box, consider keeping it out for these kinds of tasks to preserve your sharpest blade.
Now the utility knife blades are good alternatives for straight cuts -- far more forgiving and the tips are far durable, but they're a lot less friendly in turning around curves -- tradeoffs. If I'm building outside (usually at flitefests and such) they're a durable go-to, but back in the shop, hard to beat the feel of a fresh #11 blade in a good handle.