When maven says “resolution will not be reattempted until the update interval of MyRepo has elapsed”, where is that interval specified?

I used to solve this issue by deleting the corresponding failed to download artifact directory in my local repo. Next time I run the maven command the artifact download is triggered again. Therefore I’d say it’s a client side setting.

Nexus side (server repo side), this issue is solved configuring a scheduled task.
Client side, this is done using -U, as you already pointed out.