Most hosts are quite responsive and will reply to new requests or messages within a few hours. Keep in mind that they may get busy with their personal lives, work schedules, and other daily commitments, so it's not unusual for them to take up to 24-48 hours to respond. After 48hrs, the booking request the booking request will expire.

You can also take advantage of the Instant Book feature for a faster process. This allows you to book an RV automatically without having to wait for the host to confirm the request. RVs with the Instant Book feature enabled will be identified with a lightning bolt icon. 

You can read this article for more information on this feature.