J-2 visa

Hi! I'm going to start a colorectal research fellowship in September.
I have some questions to ask about the possibility to request a J-2 visa for my Spouse:
Does J-2 visa have some limitations about the time you can spend out-of-USA or in your home country?
It is possible to apply later for a J-2 visa? or only during my application for J-1 visa?
Thanks a lot for your help!