He probbly has you stay to make everything look normal. People get suspicious when they see cars coming and going all the time, I know I do. If you stay a little while it looks more like a friend is coming over and hangin out for a little while.
I don't know why that guy thinks its going to cost $75 for the fan after the mod, but he's wrong. You buy an extra 5 or 6" colar for no more than $10 and silicone and the mod is done besides the ducting you have to buy. I use one on low setting and it drops the temps in my cab by almost 20...
If you counted from the day you switched to 12/12 then you might only be at around 4 weeks now. Id just let it grow and not worry about how many days its been.