So he's in jail for 8 months for a murder.. So once 8 months has passed in game will he be released?

Of course not. This is Skyrim. Brand-Shei's supposed to be jailed for like 3 days but he'll be locked in for thousands of years if you just play that long.

Brand-Shei was actually supposed to be freed, but a bug has made that impossible. Sibbi is never released, and I don't think it was ever intended for him to be released, despite the passing of in-game time.