I've actually done this quite a bit. It actually take days for some fish to adjust there osmoregulation, but mollies are able to make the transition most of them time. These days if I have cycled brackish media handy, I do it in a spare tank over 4 days, which almost guarantees survival. With the 20+ mollies I've attempted transitioning, I've had no less die off after a 10 hour drip, than 2-3 hour drip to be honest. I actually was researching and trying this with different captive bred brackish species that had been found in full marine conditions like black belt vieja, with hopes of having more CB options for SW tanks. With those species it took weeks though. The most important thing would be while drip acclimating to add a small heater and a bubbler. Also if there is other fish, I like to keep mollies in a breeder box or huge net for the first couple days while adjust to the new environment, my clowns are usually rough at first.
Edit- I forgot to say, that is just how black mollies look when they die. I learned that from working at a not so good LFS for 3 years. Still QT newly transitioned mollies, they are more susceptible to SW diseases initially, in my opinion.
