How long can i wait to put the implants in after I did the bone graft for my four front teeth ?

Depending on the type of bone graft material utilized will determine how long you need to wait until the implants are placed.  Also, the type of material used will determine how long the graft will last.  It is not common for a graft to "disappear" after 7 months no matter what type was used.  Normal graft healing usually takes 3-4 months.  I routinely place the implants at the 3-4 months time without any issue.  Bone grafting is technique sensitive and requires proper training.  It is not always a "simple" procedure. Proper initial diagnosis of the affected area is crucial to determine what type of material and graft procedure is needed

I do not know what type of grafting you had or the difficulty of the case but in general after socket grafting you should wait a minimum of 3-5 months prior to implant placement.  It would be unusual for a successful graft to disappear after 7 months.