If you're relaxed enough and aren't focusing too hard on remaining conscious, then it should only take a few minutes or less. Where a lot of people get stuck is they're concentrating too hard, and/or not paying attention to the HI. Even after entering SP they will lie there for 30 minutes without entering the dream because they don't allow the dream to form.

Make sure that you allow the HI to create the dream for you, or you could help it along by imagining the dreamscape (this is technically classified as a VILD).