I say you can register whenever you feel like, but people most likely wont start buying stuff until they get an invite to the shower. So you can register now, but my guess is that you'd be updating an awful lot considering things sell out so often and inventory changes. We are getting married in July as well and I plan on setting my registry up in Mid Feb. and that's only so I can get it out of the way! lol
My FH and I have registered at 3 places (Sears, Pottery Barn and Belk), but I say register at as many as you want. Our 3 have very different styles, which is what I recommend if you're registering at more than one place. Give people a variety, but don't choose 2 stores that are alike, i.e. Target and Walmart. As for when you should register...I agree with the other comment. You don't want to do it too soon, otherwise things will sell out, be discontinued, etc. We wanted to get ours done before we sent out our save the dates, so that's another thing to think about.
As for when - at least a month or two before any showers, etc. b/c your guests will buy off your register for your shower.