thats not much at all. from 20k to 5k. it should not take long to bake this using the transfer surface tool in maya. also baking depends on your map size u are baking out to, and amount of samples. there are other 3rd party utilities for making normal maps, and surface info soemthing like this https://www.unrealtechnology.com/scre..._creation2.jpg to this https://www.unrealtechnology.com/scre..._creation1.jpg that might take a while to transfer....