Question..... How long after a purchase does a substantial...

Question..... How long after a purchase does a substantial holder have to post their change in holding ? The reason I ask is that if it is on the day of t+3 then if the big buyer on Monday was walker or bpt then they would likely have to post the change in substantial holding yesterday or today. So we may be able to rule them out which is interesting in itself. All speculation and IMO. But any clarification on this would be good