I once had a client tell me she’d heard that sentences should never run more than fifteen words. To this day I have no idea where that rule came from, though it was probably from someone who either had a short attention span or had read way too much Henry James.
The rule is nonsense, of course. It wound up making all her characters seem like they had short attention spans. But it shows one of the dangers with trying to write by the rules – you wind up limiting your characters or story so you can color within the lines.