In my day we would try to start to board a 747-200 an hour before if the CC was happy and they were fully ready - if not at around -45 mins before dep.
You can do it in 20-25 mins if all the pax are at the gate and boarding is done strictly by seat row numbers but frankly those days are gone -
I once boarded a 488 seat Orlando with 14J and 474Y pax in just over 20 minutes
Also at outstations or on remote stands with 2 sets of steps boarding can be done in 25 mins if you have your sh** together