I keep an extra blade sharpened and change it once a month during mowing season. I usually mow every 4-5 days.

After re-reading your post I see that I did not answer your question. I mow 5,000 square feet every 4-5 days and usually get 3 years service from each blade. I replace them when the sail has worn down about 50 percent.

Well, on my 1983 Snapper Hi-vac RER, i finally decided the blade needed changing. now, i always sharpened it every spring, so i decided after that long, an investment in a new blade would be proper.
So, for around $20, i bought a new blade and installed it, with the up-lift wings, as on the old one!
Wow, i thought--I'd cut the grass now, instead of snatching it off with that old blade!
First time out, the blade smacked the edge of the housing and bent the edge up. Fixed that. Again, bent it somewhere else! fixed it, and fixed and fixed--ad infinitum. Finally got so disgusted, i hunted up the old blade, reinstalled it, and all is well again! I can't really complain, as the dealer friend gave me the new blade! (I have given him numerous Toro parts mowers, and other makes, so it was a sorta trade-off!)
And, yes, i don't believe in buying something new, if i can keep an old one running!) Aww, it would be nice to have a big new green machine setting out front, but--i'd probably be tempted to put a for sale sign on it,. and go back to my old Snapper!

If a Lawn-Boy 'Tri-Cut' blade doesn't get damaged or bent, it can stay in service until either the end of the blade is less than 1" (the sharpened edge is upposed to be 'laid back' at an angle rather than creating a 'step' in the blade) or the hi-lift wings thin down to where they start chipping/eroding - this will most likely happen first if you do much leaf mulching. Quite often they can stay in service for many years. Safety considerations (metal fatique) will usually have me changing the blade before it starts to show serious wear.

On my 21" push mower I go through about 3 blades a year. The "wings" that create the lift wear off from the sand and then I get clogging in the deck. My mowing isn't typical. I mow about 6-8 hours per week during the growing season.

I suggest that every new mower owner buy an extra blade. Tip it up, dump the oil, put the second blade on, refill the sump and ready to mow every 25 hours. Sharpen the blade and repeat every 25 hours. Your going to go through at least a couple of blades during the life of a mower anyways.

I agree with the idea of having an extra blade sharpened and ready for swap-out every month or 25 hours. Truthfully, that's more often than I get around to it but the sharp blade will give a better looking lawn, particularly when the dog days settle in. One factor that I didn't see the others address is abuse. My grandson uses a "cut first and ask questions later" approach. He just started "helping" last summer and I've noticed a more "serrated" edge on my blades than in prior years.