Rentals of less than 12 months tend to be furnished short-term lets which are very expensive compared to 12 month lets, but include all furniture, all bills, absolutely everything. These require payment in advance. Most people who stay in our short-term accommodation stay for 4 - 6 weeks.
12 month rentals are unfurnished, do not include bills, and are paid monthly by signing a legal contract. You also have to pay a deposit. We own several of these and no-one has ever signed up for less than 12 months as they wouldn't be accepted.
There really isn't anything in between unfortunately.
BB