Agreed about the wizard making groups easier, regardless of whether or not they are minions.
I ran an adventure for some folks at level 15 (adaptation of what would've been an RPGA mod for our region if the living campaign hadn't ended and we were allowed more than 4 mods this year).
Two wizards (and a Tactical Warlord MCd in Wizard) in the party were extremely effective in keeping the foes separated and debuffed regardless of the type of fight they were going against.
The interesting thing to me was that I don't think they stole the spotlight from anyone else in the party, despite being so effective. Everyone got a chance to shine.
As for durations of the combats in 4e, once people get the hang of their powers, they tend to go around 45 minutes if fighting foes of equal level or less. A bit over an hour for fights at level +1. Fights at level +2 or higher can take up to an hour and a half. I've found this to be true from low to high levels. The highest I've played though is at level 18, so I'm not sure about epic levels, but I don't see why it shouldn't hold true there as well.