After how many days is it bad if you haven't had a bowel movement?

It depends: There is tremendous variation between the number of bowel movements normal people have each week. The important thing is for you to know yourself - if you usually have a bm every day and now you haven't in 3 days that is significant. On the other hand if you usually only have 1 bm a week then missing a day or two is not a big deal. If it is abnormal for you and you are uncomfortable see a doc.