Depending on which role you're being promoted from, it may take up to two weeks. I forget how many hours is necessary for training. If you're great at multi-tasking, then it shouldn't take you very long.
When I was promoted to GSA (this was in 2008), I was only given three days of training. But I was also doing GSTL shifts technically before the promotion, so there wasn't much for me to learn that I already didn't know. If you're new to cashiering and the service desk, then they will give you training in those areas first. Probably a day each that area. They may also introduce you to the cash office and how all that works if they want to go the extra mile.