[inceptivecss]

1) I'd argue that what they're paying for is less your time and more your knowledge and ability to provide them with answers. They're only going to complain if you're specifically charging them for an hour of your time on the invoice. If you instead are charging a fixed-fee amount for an expert consultation, it doesn't really matter how much time they take. You set a timebox so they don't take advantage of you, but if they don't need more than 10-15 minutes of your time to get the value they wanted out of the discussion, there's nothing wrong with that.

2) It sounds like you could benefit from showing initial value in some other way, such as a newsletter, case studies, or knowledge-base.

3) From a client perspective, it feels odd that I'd walk into a free initial-consult meeting and then get an invoice afterwards, even if I got value out of it. I think most clients would feel the same way. If they're paying for a consult, they'll want to know up-front. The other way feels like a bait and switch.

That said, you know your clients best. If you think that they'll be receptive to that sort of thing, give it a try.