I spent about the equivalent of 15 full labor days to do mine, full tear-off plus replace half of the plywood boards. Added ridge vents, soffit vents. All flashings and joints done to specifications. 3000 sqf roof (house + garage + front porch + back porch), 6 tons of removed material. Feels great to be able to walk up the ladder with 80lb pack of shingles and 4x8 sheet of plywood at my age all by myself. (my wife helped with the clean up).
Saved about $6-8k.
Lost 15lbs.
I go up the roof a couple of times each year to clean the debris, gutter, or just sit there to have some good view and breeze. With proper care, this roof probably will last as long as me.
Of course, I wear a safety harness when I go near the edge and hard hat when needed.
And I cut my trees too.