Bars are usually in a civilized town/city. This usually means -
1) Use of weapons in city limits lands you in trouble with the law.
2) Use of magic against others in city limits land you in trouble with the law.
3) Bars are usually patrolled by the city watch or have their own 'enforcers'.
Usually as soon as a weapon get's pulled (or spell casted), I'd figure 1D4+6 rounds for the city guards to show up. +/- a couple rounds depending on the location of the bar.
Weapons also usually mean people getting the heck out of the bar as fast as possible and finger pointing once the city guards show up.
Some bars might not even let people in wearing / carrying armor and weapons. As those might be seen as trouble magnets.
Location and cliental of the bar can play a big part of what happens in a bar fight.
Parties reputation can have a big effect on the aftermath of a Bar Fight.
Most high level character shouldn't get into that many bar fights. If they do, those same characters should easily win it in a round or two but then have to deal with the aftermath.
Low and mid tier bar fights just run as normal without using weapons or magic. If weapons or magic get's used, drop the law down on them. Judges really like fining adventures ^_^.
Only thing to worry about is if the party has a monk, brawler, or someone else skilled in unarmed combat.
If you want to be mean, mix in a level 1 or 2 commoner as they can be one-shotted, dead. Lots of trouble for the party then.