The standard length of time before a headstone can be placed is 6 months, this can sometimes be longer but that is down to the individual church or cemetery.
You can pre plan for the headstone to be made ready to be placed as soon as the 6 months has passed or you may decide to wait until after this time to start the ordering process. The grave will be marked with a wooden cross with a name plate.
Depending on whether the grave is in a churchyard or a cemetery the rules as to what you are allowed may be different but we can help with this information, please do just ask.