The pain from the needle during the injection lasts just a few seconds, but the muscle pain at the injection site is noticeable for hours. That said, I'd rate it as much lower than ericyu3 rated it. For me, this is one of those situations where having the explanation for a sensation in hand, and knowing that it is self-limiting and harmless, makes a large difference. I'd be quite concerned if I had a pain of identical magnitude but with no explanation for what caused it.