If you request an informational interview with someone, ask for 20 minutes of their time. Wear a watch so you don’t have to touch your cell phone and when twenty minutes is up, announce “I said I’d only take 20 minutes of your time and I want to keep my word. Thank you so much for your time.” If the person absolutely insists that you stay, feel free (but you never want someone you’re trying to impress to feel like you’re hijacking their time—that would send the reverse impression).
Then, don’t forget to close by asking “Given my skills and experience, are there any positions here that I could be considered for?”. If you’re speaking with someone who you know does the hiring you could ask “Are there any positions here that you’d consider hiring me for?”
If you stay true to these tenets, you won’t waste a single opportunity to locate a job and you’ll become known as someone with integrity who balances politeness with a shrewd business mind. With a little luck, it shouldn’t be long before you’re offered your dream job.