I am well aware of the fact that Lithium Ion based batteries have no "memory", and that they are able to take a charge to their maximum rated capacity straight out of the manufacturing facility, therefore there is no reason nor benefit to "condition" the battery as it were.
As for how long they can be charged for, unless you are limiting the charge rate to something ridiculously low, like C.07 (3300mAh/15 hours/3300mAh=C.066E2), 15 hours will potentially cook the battery into a thermal runaway and cause it to fail catastrophically. The charge rate of a typical consumer Li-ion battery is between 0.5 and 1C in Stage 1, and the charge time is about three hours. If you were to extend that to 15 hours, you would essentially have "trickle charged" the battery for upwards of 12 hours, and LI batteries can not stand trickle charging once their rated capacity has been reached.
Charging Lithium-Ion Batteries
As it is, these phones won't allow the battery to be charged beyond the point they reach 100% anyway (3 hours for the RAZR, 5.5 hours for the MAXX), so leaving it on charge for beyond the 100% mark is for the most part waste of energy. The charging system shuts down once the battery reaches 100%, and then stands by waiting for it to deplete to 90%, at which time it resumes charging to 100% and then shuts down and the whole process repeats itself.