i don't use carboys for primary ferment, i use open top bucket.
but carboy is ok, just a pain to use.
if your using a carboy to ferment in then you can simply let it sit on the lees for quite some time as it under an airlock. when the yeast has fully dropped then you can rack into the other carboy.
time it takes depends on how it ferments and if you degass etc.
it may take 2 weeks for the main ferment, a few more weeks to do the last little bit and degass. may take a few more weeks for the yeast to fully drop.
one problem with using carboys to ferment in is you always loose a bit when you rack, so you can have problems with topping up the carboy. with buckets i ferment more than 5 gals and keep the extra in another bottle. that way i can top up the carboy after racking.
downside to using buckets is you have to rack before it finishes the ferment, so you have co2 in the carboy.