Reporting in from Montréal:
Typical event length is about 4 hours, though at my favorite place, only start times are firm, with end times being "until there aren't enough couples left for it to be fun, and the organizer quietly asks the DJ to play La cumparsita." There, I always attend a Tuesday night that's 20h30 until around 00h30, and a Friday night that runs about half an hour later at both ends. I sometimes attend a Sunday night that's 19h15 (yeah, I know) to 23h00 (sharp, which is unusual here). And I occasionally get to other events, one of which runs from 21h00 until people decide to leave, which has been as late as 02h00 but which is generally closer to 01h00. One organizer occasionally tries to run all-nighters because he has a vision, but they always end at around the 4- to 5-hour mark.
So, 4 hours.
This is a slight change. Where I used to tend bar from 2013 to 2017, the Friday milonga ran from 21h00 until "when people went home." As the barman chasing the last people out the door quite often, "when people went home" was regularly as late as 03h00 . . . the other bartender and her husband the sometimes-DJ got home just before dawn, because they had to drive to another community. After closing and cleanup, I once or twice saw twilight on the walk home. But 6 hours wasn't the norm everywhere, only at that place.
Tandas here are still mainly 4T-4T-3V-4T-4T-3M. A few DJs prefer 4V to 3V, which everybody loves (not sure why more DJs don't do it . . .). One omits most or all of the milongas, which almost nobody likes (not sure why he does that). A few will switch from 4T to 3T if the roles are unbalanced – in practice, that means too many followers – to facilitate more frequent partner changes, but it doesn't work, because the same people who always get to dance just dance more, shorter tandas while the same people who rarely get to dance sit out more, shorter tandas.
There have been on-again, off-again "speed tanda" events with two-piece tandas, where the idea is just to dance with lots of people as a meet-and-greet. I only ever went to one. The fact that they are more off than on, and relatively rare, suggests that the format doesn't work around here.