I've negotiated pretty steep discounts for long term stays (as noted with the sales department). Typically, in these arrangements if you check out once during the stay it may void your long term stay rate (90 straight days, cannot check out on weekends).

You can negotiate some pretty good stuff like access to the executive lounge, wifi, laundry/breakfast, etc..

Also be careful in the wording of the agreement you sign as they may try and remove spg credit and give you a steeper discount.