5 hour sessions x 1.5 sessions per week since 4e was released.
Highest campaign to date was 19th level - a 4e version of the complete original Dragonlance saga. The party TPK'ed heroically against an Aspect of Takhisis (Tiamat) after making it all the way through 'til DL 14 - Dragons of Triumph. Thirty sessions exactly.
Our first 4e campaign TPK'ed at 12th level. Our new weekly campaign is only at 3rd level, but we have a monthly-ish campaign that has reached 14th level.
As a side-note, my experience is that (using homebrew and adapted 1e/2e modules) "module speed" is only marginally impacted by the paragon tier. We can usually do 6 solid "encounters" in a 5-hour heroic-tier adventure with 4 players. 4 of those are usually combats, and 2 are major skill challenges or long roleplaying segments (e.g. 30 minutes negotiating with NPCs). At paragon level, I found that our "module speed" dropped to about 5 encounters. Usually, that involved me dropping one of the four combats. That's a significant improvement for *my group* (your mileage may vary) over 3e, where I found that we could only do 2 combats per session towards the tail-end of adventure paths like Age of Worms.
Of course, I haven't seen 4e Epic level yet... and I haven't DM'ed any of the WotC 4e modules. Most combats I design involve low-hp, high-damage creatures, and I try to use the standard DM tricks like hand-waving the closing rounds of the fight once the outcome is no longer in doubt (the monsters retreat, surrender, or "...you mop up the rest without any further difficulty").