I have played through twice,
Once - 6h 30m
Twice - 4h 30m (i had specific achievements as goal though, turret + air plate, last message, Smash tv, the doors, borealis, cube catching, scanner, pit boss, refuse to do chap 8 test 1)
And i achieved all SP achievements, with the exception of 1 (70s in test chamber 10) thats the last one i try
But then i usually don't dawdle around.
That said, i enjoyed the 2nd playthrough far more, because i tried some alternative solutions (sadly all of the liquid puzzles don't have any) and i knew where to go.
So basically, 99% completion of SP takes about 11 hours. And i played another 2 hours in coop