On single-player softcore, as soon as you can beat normal Baal without dying inbetween. Also, at level 25 the XP penalties for monsters of a higher level decrease drastically. If you can beat normal Baal and his minions, you can as well proceed with questing, as you will have to do that anyway. If you have problems with that, you can still do a few Baal runs at /players 8.
Also, once you beat Normal, you have the ability to reroll maps, and to access the Normal Cow Level, which for a L25 char is perfect for leveling up, if you can handle it - just dont kill the King
The only reason anyone waits on bnet is because you cant see normal games to join once you beat normal. If there was an option to view Normal games with a NM char, people would take the quest on their first Baal run.