I was under the impression that 5 years was the norm, considering a lot of undergrad courses (in engineering especially) take about that long to complete.

Grad schools too can take that long if you're aiming for a PhD.

(Although, when you graduate your F1 expires even if there's still time left on it; so you can't continue staying in the US after graduating without obtaining an OPT, or a different type of visa, PR or whatever).