Hello @Danandnan and welcome to the forum. Having now been on the forum for some time, I’ve seen variations of your question. Having just searched with the magnifying glass (using key words time to respond), I’ll add a few here for you to review:
Unread applications and non-responses (305 responses)
Responding to sitter applications (76 responses)
How quick would you expect a reply from a sitter? (26 replies)
What this tells me is the range of answers to your questions is extensive and they are as varied as the circumstances of each member.
I’ll give my personal opinion on your final question. I always think a response is courteous. If you think the reason might help the owner improve the way they manage their listing, then mention it, in a polite way. Otherwise, a simple message is sufficient. Remember, you may want to be considered by them at a future date. 
Francine,
Volunteer Forum Moderator 