I used to wait longer but found that a read message with no acknowledgement usually means they are not interested.
If the homeowner replies with a “I’m reviewing applications and will get back to you” I’m happy to wait.
I applied for a last minute sit recently and after seeing that the message was read and not receiving a response I canceled my application. I always send a polite message wishing the homeowner well, when I cancel. The homeowner got in touch with me right away and apologized for not getting back to me. She asked if we could please reconsider and we did. It was a quick weekend sit in Northern Michigan and we had good fun with the animals.