Typical rule of thumb if you get a charger to charge them faster is you do not try to charge at a higher rate than they can contain when charged.
For example, a 1500mah receiver pack shouldn't be charged at more than 1.5A. Personally, I don't charge them at any more than 1A, which means 1.5 hours from a dead pack to a charged pack. A slower charge makes them last a while longer typically.