My Grass is What?

So the lawn care people came the other day to spray for weeds. As always, I get an invoice in the mailbox with some tips (aka more expenses) that I can do to improve my lawn. They mention about how I am watering and cutting my lawn.

On this current invoice, "Andrew" checked the box indicating that I was cutting my lawn too short. Well Andrew let me ask you this...

I haven't cut it in over 3 weeks....so if this is mowing too short, what length should it be?