Kinda depends on where you live, where you have to ship it to, how busy they are (luckily it’s not the after-Christmas rush), and various other aspects.
But generally a few weeks allowing for shipping times.
You will get a new unit. It’s not cost-effective (for SanDisk) to repair them. Your returned one will get thrown in a pile (not literally) with others and sold off in bulk to re-furbers, who will then do what they can with it and in turn sell it to a place like Woot, Froobi, Buy, Newegg or other places that deal in re-furbed products and it will eventually make it into the hands of another; maybe working properly, maybe not.