The first season of BSD takes place over five days, see here

• First episode, Atsushi and Dazai find each other, Dazai decides to take him in and help him out
• Second episode, takes place the next day when Atsushi wakes up in the apartment Dazai set out for him, then Dazai takes him to the Agency where he takes and passes his entrance exam with Junichirou pretending to be a bomber
• Third episode, immediately after episode two, Junichirou apologizes to Atsushi for all that and they go off with Higuchi on a case, Atsushi meets Akutagawa and he injures them badly
• Fourth episode, probably the next day, Atsushi wakes up in the infirmary, the Agency gets raided by the mafia
• Fifth episode, takes place right after episode four, they’re still cleaning the Agency post-raid, Ranpo gets called off on a case
• Sixth and seventh episodes don’t take place because in the light novel that’s actually the story of Dazai’s entrance exam, so it’s in the past
• Eighth episode, Junichirou’s all better so it can’t have been that long since episode three, Yosano takes Atsushi shopping and he saves Kyouka
• Ninth episode, takes place right after or the next day, Kyouka wakes up in the infirmary, Atsushi takes her to the police station but Akutagawa takes Atsushi
• Tenth episode, same day, Atsushi and Kyouka both beat up Akutagawa
• Eleventh episode, Atsushi brings Kyouka to the Agency right after all that, meanwhile Higuchi has to save Akutagawa from the people he messed with in episode ten when he’s too injured from his fight to do it himself
• Twelfth episode, finally there’s some passage of time because it’s the beginning of the Guild arc and no longer connected immediately to the episodes (then again Kyouka moves in with Atsushi)

So basically:

Episode one is one day

Episode two and three is one day

Episode four and five is one day

Episode eight, nine, ten are one day (or two days)

Episode eleven is one day

All in all it was a heck of a week for Atsushi!