In total this covers around 120 million changes to the map, by almost 20,000 contributors across 1,000 projects. This required an estimated 165,000 hours of volunteer work! There's a monthly breakdown of this activity in this Google spreadsheet: "2016-01 HOT activity timeline".
I'm keen to do an animated version at some point! Also, could a cartography geek please recommend a suitable projection for this map? Atm it's just the default WGS84, with apologies :)