The pattern for most series on the big networks usually goes like this, with a few minor variations.
Late Sept. or early Oct. - Season Premiere
Oct. through Nov. - new episodes, especially during November sweeps
Dec. through mid Jan. - reruns/winter hiatus
Mid Jan. through March - new episodes, especially during February sweeps
Mid March through mid April (or thereabouts) - reruns
Mid April through late May - new episodes (May sweeps) and Season finale
June through Late Sept/Early Oct. - summer reruns
Of course, last year Alias skipped the spring re-run season and ended early- in April instead of May. It's nice to avoid the reruns, but it also means the summer hiatus is even loooooonger for us.