I would think that it would have to depend on the area you are in and the money in the area.
I see the "average homeowner" slowing down soon as the people with more money will continue strong.
Not sure as you will have to just look around your area and see if people are treating their lawns.
Will you have much competition if you do this?
Biggest question: Do you have any experience or expertise in this area?
This is probabaly something I wouldn't get into if I didn't know anything!
Yes you can learn this stuff and probably be good within a year or two! I would make sure you know what you are doing when applying fert and chem.