I don't bother with look at end per cast, I see it as end per second, which is 15 end per 2 seconds, or 7.5 end/sec. Since the end is on the top of the fraction, the endurance reduction enhancements can be applied to this the same was as in endurance per cast, and this value can be directly subtracted from your endurance regen, which is a base of 5/3 end/sec. I'm using fractions here for preciseness, since the more often used value of 1.66 end/sec loses some preciseness as you add in Quick Recovery and/or Stamina.
Toss in stamina, at a 25% bonus base, or six slotted to a 75% bonus, that means you regen at 35/12 end/sec, which is roughly equal to 2.92 end/sec. Thus, to achieve a point where one can easily port indefinitely, you need to reach a 1100/7% endurance decrease, or 157%. This can be done with 5 endurance enhancements in teleport, or you can just use 4 and live with losing a bit of endurance on the way.
So, knowing that, here's a table with 3, 4, and 5 end enhancements, if you have Stamina 6 slotted, on how many times you can teleport continuously(no breaks), as well as the distance traveled (in yards) if you have no range enhancements:
3 enh-- 58 5800
4 enh-- 158 15800
5 enh-- infinite
Now, given that the farthest distance you ever need to travel (from one corner of Indie Port to the opposite is a good distance) is roughly 3 miles, even with just the 3 enhancements, you can easily go that full distance w/ only 3 endurance reduction enhancements. Using 4, you effectively don't need to worry about it, since you can go 8.9 miles with it before running out of endurance.
If you don't have stamina and don't plan on getting it however, you have to live with 5/3 end/sec regen, so you'd need a good deal more end red in Teleport, on the tune of 10 of them to achieve the same results. However, you only get 6, so the max possible distance you could go if you don't have any form of endurance regen increase would be 6000 yards. Still good, and actually a bit better than if you did 6-slot stamina with a 3-slot teleport.
If you have Quick Recovery instead of Stamina, then you'd need even fewer slots to be able to teleport as far as you need to go. With just Quick Recovery 6-slotted, you regen at 10/3 end/sec, and thus with 4 end in Teleport you can 'port indefinitely, while with 3, you can travel 12,000 yards before running out.
None of these calculations use any range enhancements however, and adding those would further show that it is never really needed to place more than 3 end red enh in teleport to achieve a good distance before running out.
Longest ITF was on was over 2 hours. Shortest was 25 minutes with four Scrappers, a Brute and 2 Kins. A wood chipper on rocket fuel couldn't have done much better.