Some commit logs:

DarkSoulsDev (exploring From Software files): 2015-08-25 to 2015-09-05 and then 2016-01-25 to 2016-02-27;

DuratorEmu (WoW server): 2015-11-29 to 2016-01-31;

Summoned (Minecraft engine): 2019-02-03 to 2019-03-22;

RustedIronRing (From Software files tool): 2020-04-11 to 2020-05-30;

Vatu (chess engine): 2020-05-31 to 2020-07-01 (bitboard branch);

Edmond unfinished rewrite (IRC bot): 2020-10-08 to 2020-11-09;

Some scientific anecdotal evidence:

Intense drawing training used to happen in similar sprees (around 2015-2016);

Learning to play the piano: between 4 and 6 weeks back in 2020's winter;

Learning Meshuggah's Catch-33: sometime in the summer to late September 2021;

Learning and playing chess: mid September to late October 2021;