Pay close attention. Watch for cues.
One basic rule that is perhaps the most important rule of networking... that is to pay attention to the cues you are getting from the other person.
If you have just met a new contact outside of traditional networking venues, try and gauge how interested they are in talking business right then and there. Someone you meet on the plane, in a bar, or at a party may end up being your most valuable contact at some point in the future, but if you infringe on their downtime, you may never get past go.
If someone is responsive to your business talk, that’s great, but don’t push it. If the time isn’t right, simply say you enjoyed meeting them and that you would like to swap business cards with a view to chatting about mutual opportunities at a more appropriate time. People will appreciate your consideration, and by respecting boundaries, you will have already scored some points with your new contacts.