I don't have data to back this up, but I do believe that conversion may happen a bit faster than traditional mashtun mashing. While it's true that speed of conversion is temperature dependent (a 158F temperature mash will convert faster than a 145F temperature mash), in a BIAB often times the grain is more thoroughly crushed and often a much thinner mash is employed.
I don't think conversion happens fully in 2-3 minutes- otherwise why would we bother with adjusting the mash pH at that time? I do believe it happens more quickly than usually thought, though.
It may take longer for adjuncts to convert, so I usually go with a 90 minute mash when using a lot of adjuncts in a batch. Otherwise, it's 45-60 minutes for most mashes for me. It's probably not strictly necessary.
Still, once conversion is complete, the enzymes will still be active and can further debranch the complex sugars and change the malt profile. I have done shorter mashes (20-30 minutes) with a warmer temperature mash, and done an iodine test showing complete conversion at that time. I've never gone shorter than that.