I went down 1 level in the rare earth pits. How long before I can return to my higher level?

The pit will reset after a set amount of time, depending on your pit level. This might be 24 hours, 48 hrs etc. You can see this in the remaining 'Excavation Time'.

You can also reset the pit is you are not actively gathering (you can recall troops after a certain amount of time). To do this, enter the current gathering field. Then tap the back button on the top left. Instead of tapping Exit, tap on the Reset Field button. This will start the excavation time again and let you choose a new field to enter.