How long? Oh, almost ten months! I arrived in the middle of January. Yourself, Miss Rockbell?

[[Hard to believe it's been such a long time! He's almost proud of himself, even though he's lost some friends along the way.]]

But yes, you'll find that your Riolu will only evolve if you treat her well, and don't give her any reason to dislike you as a trainer! But Pokemon have a natural affinity for trainers who treat them well, so I'd say you have no need to worry about that!