Ok, so here's the thing with curing times. Aquarium safe silicone should cure within 48 hours at optimal temperature and humidity. But who actually lives in optimal temperature and humidity, especially for 48 hours straight?
As the silicone cures, it forms acetic acid (vinegar) which is what you're smelling. Bottom line is that if you can smell it, the reaction is still taking place and it is not cured.
That being said, the quantity being produced is probably negligible for a mature tank, but I'm of the "better safe than sorry" school of thought.
Rick is right though. Silicone isn't the best choice for this. From the picture, I'd actually recommend something even cheaper that you can find anywhere. Superglue! Cyanoacrylate (active ingredient in superglue) is aquarium safe. It can even be used underwater, as it does not produce any noxious chemicals while curing.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef