This does not work as well as the gleeful bloggers who write about it want you to think (though MEN is very reliable).
Regrowing veg is a form of cut-and-come-again with a micro-hydroponics twist. You will get 2 or maybe 3 generations of regrowth before the plant is just exhausted, far less if you aren't providing supplemental fertilization. My experience is the quality with lettuce deteriorates with each generation, and after 3 cuts it's not really worth eating.
This isn't to say this technique isn't fun and novel, but you will not get romaine lettuce, or any annual veg, to "last forever." Sorry. :)