I boil for 90mins on everything, but not really for DMS etc etc... although the assumption is that they are covered as well.
I do it for process reasons.
Wort comes to a boil -> its well mixed up, so I take a refractometer reading -> now before I have to add the 60min hops, I have half an hour to adjust if I haven't hit my numbers. Either adjust the bitterness to account for gravity differences, or to add some DME or cut water to bring the gravity to where I want it. That 30mins is plenty of time to run upstairs and poke some figures into pro-mash -> at 60mins I turn off the gas for a second to take a volume reading, so I can tell if my boil off is going as expected. Then I can adjust the flame, boil length or maybe add a little water so I get my correct post boil volume.
The theory is that this way at the end of the boil, I will always have exactly the right amount of wort at exactly the right gravity - one of these days I'll even make it happen
Thirsty