Welcome to VATSIM/VATUSA! It really does depend on which facility you go to, but afternoons/evenings (as well as weekends) tend to be the busier times at most US facilities that I've seen, as people will log online once they're home from school/work, done with dinner, kids, spouse, real world flying, etc. As well, most facilities retain a team of mentors and instructors to train new students, and make sure that they're active and can cover different times. At my facility (Boston), we have controllers from all around, from Scandinavia to the US West coast, all of whom have been able to receive training and progress through the ranks.
I also recommend that you think a little about where you want to go, and then send an email to the facility if you have any questions. What kind of community do you want? Is there any location/airport(s) that have special meaning to you? Etc, etc. Many facilities have their general policies (or even SOPs) publicly available on their websites. Most Air Traffic Managers (who are in charge of their respective facilities) can be reached via XXX-ATM@vatusa.net, where XXX is the three letter code for that facility (ZBW is Boston, ZNY is New York, ZLA is Los Angeles, etc.). If you have any questions, feel free to give me a shout as well!
Again, welcome to VATUSA. I'm sure you'll be happy no matter which facility you choose to go to.
