Molting lasts about 4 weeks and then they start laying again. If they are not Winter layers, then give them supplemental light and they will lay all winter.
Two year old hens are too young to cull.
I have had poorly producing birds molt for 5 months (REALLY, losing and slowly slowly growing new feathers this whole time) and then take 2 more months off before laying again. Molts can be awful things on badly bred birds. Just to clarify, average molts are 1-4 months long...
2 year olds are not too young to cull, especially if you're buying from hatcheries, and especially if your management is to have yearly replacements.