The length of the fight depends on lots of factors.
I use light tippets, down to 8 lbs on adult salmon. We've mentionned 1 minute/lb, ... I guess that pretty well sets a norm that sounds about average.
IMHO, water temp. makes me decide how long. I like light tippets as I'll either bring in the fish or I can break off pretty well when I chose to.
From experiece on my home waters, most 16-18 lb salmon take about 20 minutes max. One particular specimen did take over 30 as I could actually see him bleeding and knew I'd keep it and was on 6 lb test.
That was a weird feeling, ... fishing (concentrating) almost frantically to not lose the fish 'cause I knew I'd keep it.