My LFS has no strict policy, but they've always given me the value of fish back if I need to return them (even if it was a couple of weeks to a month after) because they know I'm a good keeper. Admittedly, this is store credit, but in the worst case I'm giving them back a fish that's been fed and taken care of at my expense.
They've even let me "rent" (more like borrow) fish. Two times now I've picked up common plecos from them to eat some glass-growing algae, and they've let me return them months later and slightly larger for store credit for what I originally paid.
They're awesome!