You're fine leaving it in for a while, it won't hurt anything. Purigen binds organics whereas bio pellets serve as a food source for bacteria, so they both function differently. Once bacteria establish on the pellets and start to consume them and excess nutrients, they'll start sheding a film that your skimmer will then remove. At that point you should start to see a nitrate reduction from the pellets. (Usually takes a few weeks to month to kick in.)