Only auctions can run for 7 days, but if listed as BIN it cannot be changed, you would have to end and relist.
You can list as a 7 day auction with a BIN price, but the BIN price must be at least must be at least 40% higher than auction price and as soon as there is a bid, the BIN disappears, so not suitable for what you want.
Just end and list as a 7 day auction at the minimum price you would want to sell for.