One at a time or all at once? Take your pick...

If you want to give the fretboard a clean/polish, maybe the frets too, take them all off.

If you are doing a quick change, I'd do them one at a time. Easier to tune the new string to pitch if you've got the others there to tune to.

One at a time is a definite advantage for instruments with a floating bridge too as the remaining strings hold it in place.

I've never had an instrument expensive enough to worry about maintaining tension on the neck or soundboard!

20 mins sounds very reasonable to me, especially for 1 at a time.

Just my $0.012 / £ 0.0129

Pete