Milling is tough on chains and saws ... all that fine dust.
I usually set up two 660s with 36" full comp square file.
Say I'm milling 22" Red Oak at 7.5' length, I'll fill the tank after two cuts and change saws after 4 cuts (if I don't hit anything).
By then I'm usually spent and take the stuff home and sharpen it.
A lot of work, but I love how the boards come out and what I make with them. I usually keep the half round sides for benches.
This is at my cabin. The bench on the left is Chestnut Oak, the table is Chestnut Oak and Red Oak (the center piece and legs), and the bench on the right is Hickory.