It'll vary, but for some of the cases, we might be doing something like loading a customer's historic data, which might be on the scale of hundreds of thousands of rows.

Ok I think that the best answer would be to do the inserts for anything on the left of my sliding window, rather than to figure out how to copy and switch the partitions, as there's no quick "mirror this file" kind of functionality built into SQL server, and I would likely have to load the partition using conventional means.