I got a large chest piece on half of my chest a couple months ago. I waited till I was 2 years post op (the different tattoo artists I asked, all suggested that the scars be atleast 2 years old).
Also, I wanted to be extra careful since my scars all stretched out. The widest parts were upto an inch wide :/
The tattoo I got done in one 4hr session, I designed it so that it only went across my left side (didn't want a full chest tattoo, but wanted to camouflage atleast one scar).
It did hurt, and this wasn't my first tattoo. However, it was the area around the armpit, nipple and on the chest bone, not on the scar, that stood out as being the most painful. I was anxious about how it would feel going over the large scar, but by the time he got to that area, my skin was already feeling hot and in pain so it just blended :p