There's no set answer to this, and in the end, it's something you have to make a judgement call on, which is where experience will come into play. I always try to engage the prospect over the phone or email for as long as I need to suss them out and work out what stage of the buying process they are (10-15 minutes, or a couple of emails usually). I also try to assess whether we'd be a good 'fit' and whether or not I can actually do what it is they want, at a price they'll be happy to pay. If they tick all the boxes, we can spend more time talking, phone, face to face, whatever seems suitable.
Do you talk to them over the phone, or do you see them in person? I really have to change the way to I do things in order to progress.