For longer rides you'll appreciate either padded shorts or a padded liner to wear under unpadded (or regular) shorts. Hubby switched from denim shorts to bike shorts after only a few months of riding because he disliked the feel of the looser denim shorts rubbing the top of each leg with each pedal stroke. I wear padded liners under unpadded bike pants, or under jeans if I'm out for a short ride around the neighborhood. I tried a short ride once without the padded liner and it was very uncomfortable.
Edit: That said.... you'll know when it's time. If you're not uncomfortable yet, you can wait until you ride a little longer and you start to feel discomfort. I make most of my bike-related purchases in that manner.... when the need arises, I get it. My next purchase will be shoes because I don't like the grease stains on my everyday running shoes.
