In general, the more frequently you work on your nails, the less there is to do each time. I do it as many times as I practice. Practicing without prepped nails can be a waste, or worse. I'm still experimenting a bit so the file stays handy while I practice.
I use a glass file. They're very fine, but also can work aggressively with the right touch, which saves time. I don't always polish, though I should (to prevent accidental snags at least). For polishing I use a fine grit wet/dry sandpaper, front and back. I keep a small piece in my wallet, too.