Here's the test: If you were laid off today, who would be the first ten people you'd reach out to for advice, or to catch up with? When is the last time you spoke with them? If you can't even list ten people, much less remember the last time you caught up with some of the people you would normally call references, it's time to start reaching out now, while you're comfortably employed and don't need anything in particular.
Ask them out to lunch to catch up, or see if they can grab coffee with you sometime, just to see how they're doing, discuss your respective careers, and see if they need anything from you. Remember, a "professional network" is really just career code for "friends willing to help each other professionally," so stay in touch and see if you can lend them a hand as well. After all, what goes around comes around.