As mentioned in the earlier answer, any mower use, even in long grass without rocks, will dull or blunt the blades. If you do as you suggest, then you may not need to sharpen the blades every 25 hours, instead every 50 or 100 or 200 hours.
Worth noting that sharper mower blades will slice through vegetation, such as grass, with greater ease, which has the following benefits…
Blades
- less friction / resistance, especially in thicker grass,
- more consistent cutting speed,
Motor
- less strain on the motor,
- prolonged motor life,
- lower fuel consumption.
All these benefits translate to less money spent by you and less time spent mowing.