I got mine with a 5K iMac. I took it out of the box on January 13th and it hit 10% on March 30th, having never switched it off. That last 10% seems to have gone quickly, however, as on April 1st I noted it at 5% and now on April 2nd I see 1%. So I’d recommend the day you see it hit 10% you plug it in that night.
Note that this was out of the box, so may not have been a full charge, but I think it’s safe to say it lasted 11 weeks in my case.
