Since Instant Booking is no longer an available feature for your listings, you cannot add a heads up time for your rental items. If your rental item(s) are not available for certain dates, then simply pause your items or activate holiday mode.
It the request is too last minute, then you can simply decline the request and suggest alternative dates to your Renter. Alternatively, the rental request will auto-cancel once the requested date has passed. It's up to you!