Difficult indeed but I like where you're heading.I've been playing with this idea. It's very difficult to get right (see previous post/explanation from earlier on).
While it would be nice if the entire process could automated, it will probably need a way for the shop owner to both set which items they treat as consumable and also allow them to select the interval for each customer since that is going to vary by customer as you've pointed out.
For some more scary thinking.....maybe those types of selections can be made as the shop owner is processing the customers order. Clicking on the item on the order could present them with an interval dropdown or something like that and also set up the queued email. Saving the information might even allow the shopowner to set up an account module so the customer can adjust the interval on a going forward basis. To automate it some, you could use some default settings as set by the shopower on the products page and allow the customers to control the intervals and even decide if they would welcome those reminders in the future. The queued email could mention that.
Dan