it depends...
a lot depends on... who it's for and who is with me.
then it's... how much do I know about this seat? When we drove to Richmond to get the Regent (yes, it was an ordeal that led us to do this) for Damian, I had researched so much I was dreaming of installing these... on top of it not just fitting in the van without taking up a chair and needing to get out of the box to do so, and then, add 4hrs of having heard Damian complain his butt and back hurt... I (IN THE POURING RAIN) fit him super quick, slapped the recline bar in, loosened and clipped on the tether, installed it long path, tightened the tether, and got him strapped in.
OH... there was never a boy so happy.
When we got the triumph for Ruthie, it was big and heavy and she was only 2mo old... I had to put it between the boys' seats in the old buick... that thing got tied into the front seat (kid you not, I tied it in)... and good thing... took me awhile to get it to recline... once I figured it out, it was easy.
in fact, come to think of it... that Regent was the first seat I'd felt confident enough to just put in and go (well, except for a booster... those are easy)
I would not do it with a Nautilus, though... that's too much work, even without doing the armrests.
it's not the length of time, but the space you need to put it together.