Depending where you live, your rates seem really low. A demanding client can request 1 task every 2 days, or 20 tasks a month. Even quick tasks take an hour to complete (usually because clients don’t really know what they want). So you could be working for less than $10/hr.
If you look at any of the subscription design services, those guys are charging thousands per month.
I would limit the total time spent per month to x hours, or increase rates
I do WordPress theming (freelance, and at an agency). I don’t give fixed prices for projects, but I do give an estimate based on how long I think it’ll take. If the scope is massively different, I chat to the client about it and we come to some sort of agreement. If I overshot it, I will usually discount my final invoice, especially if it’s a regular client.