I have a rule that I'll never turn down a 30-45 minute catchup with someone who needs some advice, or wants a sounding board, or just wants to talk about something they're excited about. Life is about people, and has a funny way of turning things around so that those kinds of meetings grow into friendships, business connections, and sometimes of course - nothing at all.
Most people agree that mentor-mentee relationships benefit both parties equally well. I've definitely found this to be true. While these kinds of interactions may not be the same kind of dynamic as a long-term mentor relationship, spending a small chunk of time can also break you out of a rut or give you a new idea.
Don't discount the power of taking a few minutes out of your day to pay it forward.