I haven't let mine get to a cut-off, and they've been coming off around 3.6 or so. Typically pulling them after about a day, which is about 5 mL. I'm using a Cisco 2.0 ohm 306, and running at probably a relatively low voltage, I'd guess around 4.0, but don't have an under-load meter to test with...
I haven't felt like I had to change mine, but have for convenience (like I was going somewhere, and didn't want to carry extras, so went ahead and put in a fresh set).
For what it's worth, I don't particularly care what my voltage is! Just found what I liked...