Stock M201s, at 3.7v, last a long time. 4.0 to 4.1v = about a week. If you go over that, you can measure the life in days.
I'm currently puffing on a M201, that's been re-wound to 3 ohms, with the next thicker gauge of nichrome. It's my primary, one-and-only , heavy user... SINCE NEW YEARS. First week or so at 4.2V, then 4.6 to 4.8 volts ever since, without any signs of slowing down. (using M201 carts, no dripping)
Modded, these atomizers kick serious ...