You can do it any time and be protected by the law. There's other things to consider though. You need to get a certain number of hours (consolidation) after Ioe to not have to through training again when you get back. Also taking mil leave during probation can extend your probation at most airlines. If you have a course you have to do for the guard/reserves to get spun up though, I'd tell them you could do it after consolidation. (About 4-6 months after hireing.)