well it depends. what horsepower lawnmower u have. if u have 6.5 HP then its 6.5HP but if your using easywalk or self propelled lawn mower its about 6.4 or 6.3 hourse power. It drops when using those features cause it puts more load/work on the engine.somewere around there but once it goes back up to speed and the wheels are at max speed then the HP will come back in some engines. if your using a ride lawn mower it will change too.
Oh and it does not matter if its running for 20 min or for an hour. its not gonna loose hoursepower.
