@Siena5 Listings which take long term reservations say so in their ads. Some places only have 2 week maximum or some other limit. So it's not appropriate to ask a host who has a maximum set to rent to you for longer than that.
Hosts who do long term bookings usually have a discount already in place for month long bookings. It's considered rude to request more of a discount than the host already offers and experienced hosts also consider guests who ask for discounts to be red flags and you could well be declined on that basis alone.
As far as renting month by month, that's something you'd have to discuss with the host. Most wouldn't hold the following month for you on the basis that you'd decide if you wanted to stay longer when the time came, as they would be missing out on potential bookings if you decided to leave.