When you are more concerned about the span of time, use the -ée form. If you're just counting days, mornings, years, etc., use the normal form.
Je t'attendais toute la journée. -- I was waiting for you all day long. (= duration)
Je l'ai vu il y a deux jours. -- I saw him two days ago. (= counting)
That said, there will still be cases that don't fall easily in one or the other category or get you confused. But the rule above works most of the time.
