That's strange. The registration should not have any expiration at all. It might be something weird in Impulse, but Stardock always lets you download your registered products whenever with no limits/expirations (the only exception is the Object Desktop Plus license, I think, which really is renewed yearly).
Actually, I'd suggest you post this in the Impulse forum, rather than Sins, so one of the Impulse devs can pitch in.
