Never, honestly. There were times I felt a little tighening maybe 3 weeks before my due date, and I guess those were BH, but I never had any painful contractions until I went into actual labor. I believe it's normal to feel them several weeks before your due date though.
My first baby I only had them once for like an hour and it was a week before he came. My current pregnancy Ive experienced them from 16wks for different lengths of time and they have been slightly annoying but not painful.