I guess until they pull out? Depending on "why" you're selling, could you not just get the sale done if it's a solid price and move into rented?
For me, once I decided I wanted to sell, we agreed that if we got a sale that wanted to proceed quicker than we could buy, we'd move into rented. This meant that we could a) hold out for a good price for ours if we weren't in a chain (though this ended up backwards in the end after our buyer pulled out and we had to take a lower offer to stop the chain collapsing), and b) be in a very strong position if we had moved into rented that we could move extremely quickly as and when something did become available, and potentially get a better deal on that vs someone who had a slow chain.