Answer:
about three weeks
Step-by-step explanation:
You may wonder why it takes only 3 weeks to make a habit of writing a journal. Well writing a journal becomes really interesting at first but as you go on writing it you feel more bored about writing about the activities you do everyday unless it is about a special day. So due to this in 5 days you get pretty bored writing the same things again and again. But in three weeks you will have enough time to customixpze it to a habit.
Sorry if this is wrong but it is according to my calculations and research