If it's a totally self service place (including being encouraged to bus one's own dirty dishes) like the breakfast buffet at Hampton Inn or Residence Inn (or in a lounge where the service is nothing special): nothing. If there's some service like bringing coffee and/or juice and removing dirty dishes: $1 (per person). For a fancy buffet with some service: $2 per person as the norm. Certainly not 15-20% of the bill.