What types of tasks are you talking about and can you actually acomplish anything useful in 30 minutes. Examples please. Are you talking about setting up AdWords campaigns? Reviewing the <meta> tags on my website? Helping me refine my headline? Or will you help me set up a marketing strategy and research my customers? The latter can take days or weeks. Can I have you do that under the unlimited plan, 30 minutes at a time?
Also, "Ideally a request is about 30 minutes of work" is a pretty terrible way to phrase what you are saying. As a customer, I probably have no idea how long something takes you. I know what I need to acomplish, and will dump a crapton of work on you for the low price. Without examples, I will simply ask you "I need 10x the traffic I have now to site X" and expect that you will deliver.
I think this is a really aluring idea, but I need more details!