When I first started out I would mentally compare a new lawn I was quoting to one I already knew how long it would take. After a while, especially after quoting some too low by accident, you will get good at just being able to walk a lawn over and come up with a price. Be sure to allow extra time for slopes, lush lawns, obstacles and those with lots of detail work. I have a lawn that's under 5,000 sq feet that takes longer than lawns 5 times that size due to all of the tight spots and detail. And don't forget to discuss frequency of mowing with the customer!!!!!!!!