How long does it take to get a UK student visa?

My American nephew was accepted to a school in the UK and needs a T4 General student visa. The classes start in Sept, how soon should he begin the application process? He has an American passport, and the starting point appears to be with filling out a UK visa application at the UK website. How soon before his classes start can he complete the application, and how long will it take to get a Biometrics appointment? I appreciate any help with this. Thanks.