First 6 weeks will not get a lot of attention, maybe in the slow time of the year they will be more receptive. More importantly what are you trying to get, since look at the bigger picture. Ie just a lower rate versus things that are easier to get and make life better. I frequently go to a hotel for 6 to 12 months initially Monday through Friday every week and then will transition to every other week as it winds down. Things I prefer to negotiate.

Room lockout meaning I have the same room every week and they will block it over the weekend so I can leave my stuff there. Makes like a lot easier. Every now and then there may be convention etc and they will store my stuff.
Free breakfast, not the Hilton credit but a real breakfast
A couple of free drinks and an appetizer
Free parking
Free laundry. I had one place I gave them my clothes, when I left and they would have them hung up and ready in my room Monday.

Like I mentioned it is not always the cheapest rate but they have more flexibility in some of the other things.