2018/19 Alta MXR real world MOTO times?

I think you will need a generator. I believe you would run out of power mid second moto.

While a 240v is nice, you would get through that type of day with a small 120v generator, no problems.

I run vet juinor, and with a 120v generator I can do 4 motos and practice without issue, and I'm on a 2017.

20 minutes for a moto would not be a problem. I can get 40 to 45 minutes at a good practice pace in map 4 before it goes to limiting mode.
At Mammoth MX in the 60 class it went 6 laps on a third of battery.

I raced a Vet A class local mx in CO and probably could have squeaked out both motos and practice on one charge as I used about 1/3rd battery for each (4 lap races), but I charged in between. Ran map 4 on a 2018 MXR. I typically get a full hour of ride time at tracks out here in Colorado (per the hour meter on the bike) when I go practicing. I normally toggle between map 3 and 4. Talk to your local track before hand, some are willing to let you charge there, and you can get a 110 plug from Alta. Pull the trigger, it is so fun I sold my 450.

In my back to back tests on the Pala Vet and Am tracks between my 450 and the Alta Redshift MXR, i was consistently faster on the Alta. Most noticeably late moto.

Get the harbor freight Predator 4000/3200 generator and pop it on a charge when you come off the bike. It'll charge 1/3 battery in 30-40 minutes; less than the time you'll take catching your breath and getting ready for the next moto.