airburkina, if you don't manage to pass the first round interviews, from what others have experienced, generally they are happy for you to reapply (some people even get asked to reapply later on) - generally its atleast 6 months after, but 10-12 months has also been mentioned.
as for reapplying and the chances of getting called again, well i guess it really depends on how much they think you have improved from the last time based upon what you have written on your application. For example, if you applied the first time with say only a few hours flight experience, they'd probably expect you to have alot more hours the second time you reapply, offcourse how many extra hours depends on the time length on which you reapply.