Yes, I agree that 6 months is way too long for a rental agreement. I know of only one brokerage firm that routinely asks for 6 months.
The shorter the contract, the better for you, because then you can change agents if you are not satisfied, and it puts pressure on the agent to get the job done. The longer the contract, the better for the brokerage, because then they can use your listing to attract new business, and they've got you locked in so they can take their good old time getting a deal for you.
It is a very, very simple thing to renew an agreement like this for another month.
In your shoes I would insist upon a 60 day contract. If they balk move on, plenty of agents (me included) will sign for 60 days or even 30 days. But surely they will accommodate your wishes.
Licensed Real Estate Salesperson
New York, NY