Cargo Bikes

I don't know about the difference with an ebike, but I get just under a year from my belt drivetrains before they start growling. But I'm working 7 days a week. You should get a lot more if you're only using it for a bit each day.

Every 3 years for non-commercial use

I would swap front to back, then left to right every 2k or so, helps them last a bit longer.
On a mid drive cargo maybe 6k to 10k until u you start getting the noise depending on use. More if lighter use and less stop /starting.

what is the interval between swapping the gates belt drive on an ebike?

My shop is in Seattle and we see them get about ~12,000-13,000km on bikes w/ Gen4 Bosch. Can't speak to areas with more salt/snow, tho.