After dental bone graft, how many days will bone particles fall out?

Depends the method. As a periodontist, I do all types of bone grafting. When grafting is done after a tooth is removed, you may express particles of grafted bone for several days. If the grafting was done for another reason, you may have a slight complication. Call the surgeon and ask.
Day , just a little! Small amounts of bone graft particles may be noted immediately after surgery. Beyond that, the membrane may not be retaining the bone particles well and the procedure may need to be repeated.
Resorbed. Bone graft material typically will "not fall out" but instead act as a matrix for the bone in your body to use by resorbing it in producing new bone. If bone particles are "falling out" then it may be that the bone graft was not fully secured by a resorbable collagen membrane or primary closure by sutures.
3-5 days. Sand like particles will sometimes appear in the mouth following a bone graft. Do not be too vigorous in rinsing or excessive lost may occur.
Shouldn't fall out. Bone particles aren't suppose to fall out. They act as a scaffold to allow you bone to grow into the socket, helping maintain bone width and height for implant placement. If you feel gritty particles in your mouth, call your surgeon.