Wait time for scan after extraction?

Once an upper molar tooth has been extracted a 3D CBCT scan is only necessary if you have decided to go ahead with an Implant treatment solution. 
Generally during time of extraction the doctor will gather enough information to determine  the future need for bone augmentation or a sinus procedure. (not always) A simple 2D PeriApical Radiograph will provide enough information to at least get a preliminary idea of how much bone is available Vertically (to the sinus floor) and during the extraction procedure , visual confirmation of the amount of horizontal bone volume. 
In Implantology we often prefer to prevent the need for bigger grafts by preparing the site to best support an Implant at time of extraction. This cant always be done, especially if there is infection present but if not then ridge preservation procedures are recommended to keep the bone volume maximized until time of Implant. 
Depending on how the extraction site was prepared at the time of tooth removal (if a graft was placed, or not, and what type of graft) the wait time for a CBCT can range from 3-4 months to 6+ months. 
If it has been determined that a more significant Sinus grafting procedure is required you will generally end up getting a 3D CBCT scan prior that surgery and a follow up scan approximately 6+ months later. This second scan would be to assess the success of the Sinus graft and plan the proper positioning of the Implants. 
In dentistry we try to keep radiograph exposure to as little as possible to timing is important on CBCT scans so as to not have to redo them more than absolutely necessary. 
Hope this helps answer your question.