Doesn't matter what you use, a ZT, push mower or the tractor with a finish cutter, how long before you sharpen the blades? Assuming you haven't hit something other than grass.
Also how do you sharpen yours?
I sharpen lawn mower cutting blades once each year in the fall during annual maintenance.
For years I used a file for lawn mower blades, both walk behind triming mower and 5' ahd 6' MMM decks for Kubota CUTs. I would go through at least one file each season so last year I decided to use flap disc on 4.5" angle grinder. A good 14" file now costs about $15 and HF flap discs cost less than half that and last multiple years.
Unlike most (all?) I dress my rough-cut mower blades after each use with a 4.5" angle grinder. I use my rough-cut mowers (5', 6', 15') as HD finish mowers and almost never hit anything with any of them aside from grass and the occasional stick or branch. Sharp blades cut better and use a bit less fuel.
I find it easier and less time consuming to dress the blades after each use rather than performing heavy grinding once or twice each year.
Simply raise mower, reach under from behind with angle grinder and dress blades. Takes about 10 minutes for single spindle mowers and about 30 minutes for BW, which is used only 2 or 3 times each year. Technique is a bit different for BW but still quite simple.
Yes, I go through rough-cut mower blades faster than anyone else due to the routine grinding.
No, I do not cut saplings with sharp blades. I keep a spare set of dull blades for the 6' cutter for the rare occasions when I need to cut saplings. Haven't used them for over 10 years but keep blade pins well anti seized so changing blades is not difficult.
SDT