AOE2, a basic 1v1 skirmish takes about 60-70 minutes.
AOE3, maybe 20-30 minutes.
What’s the ballpark time on a skirmish in 4?
I’m wondering if it’s more like 2 or more like 3.
In my personal experience from 15 to 45 minutes. There are many factors that influence length of the match
most matches will last between 10 to 30 minutes
Well, if AoE2 takes 60-70 minutes for you (real minutes or ingame minutes?), then probably you should expect the same, maybe a bit less, from AoE4
But usually I’d say the 15-45 minutes estimation from @UranicJohnBeast is pretty good.
if you know how to defend in the early ages, and so does your opponent, most games go to 40-45 minutes.
1 out of 10 goes to an hour, I’ve had 1 game out of +300 which went to 1,5 hour. 
99% of the games are done before 1 hour hits the clock.