Depends on the TF. Most of the newer ones run 1-1.5 hours on an average team. Some of the older ones could take 2-3 hours. Then there's Quaterfield...
http://www.fimfiction.net/story/36641/My-Little-Exalt
Anywhere from 30min. (Katie Hannon) to 4+ hrs (Dr. Quarterfield).
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On an average team, the short TFs (Hess, Katie) will take an hour, hour and a half. The newer TFs (ITF, LGTF) generally take 2 to 3 hours. (I've never understood how people do it in 45 minutes.) The surviving 8 TFs generally take 2 to 4 hours. (Numina and Manticore tend to be the shortest.) Plan on at least 4 hours for a Shard TF - up to 6 if you want to do Dr. Quaterfield's TF.
...I forgot what experience means.
Or get with a team you can count on to stop at a certain point and pick it up again the next day or whatever.
Strike Forces?
I don't run them often enough to put in a good estimate.
www.paragonwiki.com is a great source of information for this game.
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The Mentor Project: Part of the New Player Council.
Most of the strike forces are relatively short, 1-2 hours.
1-2 hours isn't bad at all. Especially on a weekend. Thanks all.
I did three Sister Psyche TF's yesterday with three different toons. First two only took a little over two hours, third one took 3 hours and 13 minutes. We were enjoying ourselves with the XP and didn't feel a need to rush through the TF.
I wound up on a pretty awesome PUG for an ITF the other day and my jaw dropped when I saw that we had finished it in 57 minutes. I think the fastest time I had before that was a little over 90 minutes.
With the proliferation of invis + tp to short cut missions, not many players do the old school slug throughs anymore. Even with the new, already short TFs.
Teams are the number one killer of soloists.
What the above posters said is fairly accurate, IF you have a full/good team, at other times you may not be so lucky.
For example, a friend and I run TFs all the time and most of them go pretty smoothly. But sometimes people drop or the team just can't seem to work out a good strategy and everything turns to crap.
As a matter of fact, we did Positron this past weekend (all PUG) and did okay up until the third mission when half the team quit for one reason or another. Basically we were left with 3 tanks and a Trick Arrow defender, but decided to tough it out until the end. It took us 7.5 hours to complete... hardest 64 merits we've ever earned (especially with the new boss spawns). LOL
I also have many horror stories of failed ITFs, but I won't go into that. 
I remember task forces basically taking up a Saturday. I can't do that anymore. How long do TFs take nowadays?