Supernovae become super bright super quickly, and then taper off. This is why there is a lot of interesting science to be done right at the beginning: they are hard to catch. Depending on many factors they can stay visible for months. The best way to visualize this would is with a Google image search for "Supernova lightcurve" https://www.google.com/search?q=supernova+light+curve&tbm=is...