We have to disagree.
First of all, you simply cannot binge-watch the entire season/series of Firefly and not cap it off with the show-stopping, stunningly dramatic, cinematic conclusion that is Serenity. For shame, Nielsen. So for that we'll have to put the actual binge-watching time as: 12 hours
As far as Buffy goes, at this point in the game. why would you bother doing a marathon in this particular era and not cross over with Angel? Come on! Do the thing right. And feel free to use our tried, tested and true Buffy/Angel crossover guide to do the deed. Total hard-core watching time: 7 days, 14 hours
And they left off the under-appreciated, red-headed stepchild that is Dollhouse. If anyone is ever going to binge-watch that sucker (including the unaired-but-originally-intended-pilot and Epitaph One, the proper season 1 finale) the total time would be: 19 hours
Check out Nielsen's full Binge-watching post which includes many other types of shows (including Supernatural) here.