Curious about any rule of thumb for a discount to be applied for pre-payment of rent for 6 or 12 months in advance?

I have an offer to negotiate a rent discount for one unit in a duplex for pre-payment if either 6mos or 12mos. The unit has been on the market about a month with medium level interest, mostly phone calls generated from the sign posted in the front yard. I've already reduced the figure I'm asking by $100/mo due to lack of positive responses/requests for showings.

This offer today has me very interested in order to get tenants in cash flowing.  But at what discount is it advisable?  1 month free, 2 months? 

Asking rent is $1700/mo currently