The 3 free months sound too good to be true so I had to call and talk to couple of reps (including one from the leasing department) to be sure. I pretty much double-checked the following info w/ the reps and they all agreed w/ me. Hope this info helps you w/ similar situation.
1. Your 3 months credit will be applied right away - first 3 months after the original lease expiration date
2. If you terminate your lease extension before the 6 months period, you'll have to pay the 3 months credit back. This doesn't apply if you terminate your lease to get the new 2018 LEAF lease. If you terminate your lease to get a new leaf lease, you don't have to pay back the 3 months credit even if you haven't keep your extension lease for 6 or more months.
A. My 2015 LEAF lease ends July 2017
B. My lease allowable millage is 18k miles / year
C. My lease payment is $370/month
With the lease extension offer, the following can happen:
i. My August, September, and October payments are $0
ii. I can pre-lease the 2018 LEAF in September 2017, vehicle will be delivered in March 2018.
iii. I am allowed 1500 miles per month (because of my 18k/year original lease)
iv. If I don't like the new 2018 LEAF pricing or for whatever reason, I can return the vehicle in January 2018 "no questions asked". This implies that I only need to pay for 3 months: Nov, Dec, and Jan and got Aug, Sep, and Oct for free (a $1110 saving)