If you have a setting of maximum 7 days, then you should stick to that. Just message back to the guest "Hi XX, I cannot accommodate your request for a month-long booking- as you can see from my listing information (did you read it?) , I only take bookings for a maximum of one week."
And yes, a maximum of 7 days is a good choice for a new host. You don't want to get stuck with a bad guest for a month, plus you want to build up some reviews, and that happens faster with short term stays.
Don't be too eager to offer discounts, either. Be aware that discounts are cumulative- if you have a 20% "new listing" discount on, and then a further 20% discount for a week-long stay, the guest would get a 40% discount and you could end up hosting basically for free.
Make sure you understand the difference between an Inquiry and a Request:
Inquiry- choices are Preapprove or Decline. But you don't have to do either- you can just message back the guest within 24 hours. So if the Inquiry is inappropriate, like asking for a month stay, just say it isn't possible.
Request- choices are Accept or Decline. You can message with the guest, but you do have to either accept or decline within 24 hours.
@Mónica217
