I truly believe that only about 1% of people have the talent to run a very fast 400 (sub 50 for men/sub 55 for women). Maybe about 5% of people can get sub 52/sub 60) but as for everybody else...
On average, if everybody in the world trained to their potential starting their teenage years I’d say:
Men: 0:55 - 1:05
Women - 1:05 - 1:15
Obviously you would get people slower and you’d get people faster, but I’m talking about the 30th and 70th percentiles. Remember this is for average people (which includes all body types). Obviously the people with natural athleticism will run faster (20th percentile)
Let me explain why I have come to this conclusion reguarding times. 400M time is largely dependant on 100M time. I think the average male could be sprint trained to a 13.0 100m if appropriately speed trained and would hit about a 1:00 400M if appropriately endurance trained once developing the speed. The same would be true for women at around 15.0/1:10.
Obviously there are a lot of talented athletes on this website who would disagree and say these times are too slow but what you have to remember, top end speed is very limited and largely dependant on genetics. This website is jam packed with genetic freaks who sometimes find it hard to understand that not everybody can be trained to run 11s and 12s like such a large number of people here can.
Heck, on the opposite end of the spectrum there are probably some very untalented sportspeople out there who would consider these standards too fast.
Currently, the average male untrained but in their prime (20 y/o) runs a 100M 15.0 and the average female untrained runs 18.5 - this can only be trained a limited amount (by about 2-3 seconds at best). The average person is 50/50 fast twitch/slow twitch.