I think the best guideline is to think how it will affect your plot. Basically how will this affect the story and what do you want to reveal about the character in question? For basic NPCs I don't see it climbing over 3 but for most antagonists I can see it going to 5 or even 6 in some extreme cases. Keep in mind using the Sight the wizard sees things as it is. The only thing holding them back is if it is too alien or simply to horrible/vast/ect for them. Other than that well I point you to the default answer: " I dunno whatever you think works."
