Valves should be adjusted on the first service and perhaps the second by which time the engine has run in.
Under normal running you should not have to touch them for a very long time after this.
Different makers specify valve lash checks at intervals of 200 to 500 hours.
On side valve engines, you rarely ever need to do an adjstment.
Now days, to meet EPA specs, valve adjustment is very important because the emission standards are rediculously low for occasional use engines like mowers.
Thus makers emphasise the need for extra care in maintining the correct valve lash as a slightly tight valve can cause an engine to be non compliant.