I think it isn't day length, but day speed, because time goes faster if I set it to 150% and slower if I set it to 50%. I would anyways prefer it go like from 30% to 400%, because in early days, especially when restarting from the first day, no events happen and it gets really boring to just sit doing nothing, having all the tasks completed. Even on hard++ difficulty, at least for me, it takes about 8 to 15 minutes to run to the satellites without even using ATV and get all the drives, and other 20+ minutes I just wait for another day