Get some Frasca time if you never flew it before, but PCATD will do too. Study the Gleim IFR written and multi aero dynamics. Might want to read up the gouge on the HR stuff at aviationinterviews.com. Good luck, it's a great company. I loved the atmosphere, cant wait to find out if I am in.
I believe the call out for class is usually within a month if not only 2 weeks. We are so hard up for qualified pilots I wouldn't be surprised if they asked you to interview on a monday or tuesday and asked you to come to class that wednesday(i think that is still the start day for classes?). In all reality though, 2 to 3 weeks seems about average.