Look through the bibliographies of the books that you've looked at so far. Also, check out Jstor for book reviews. I plugged "Plague", "Black Death", and the other relevant search terms for my T&S into Jstor and came up with a wealth of useful stuff.
Over the summer, anything more than a month or so is probably too much. Some people do far less than that; some do more. I suppose I spent about 6 weeks on it, because I couldn't work at first and hadn't anything better to do! Plus I really enjoyed it, and the enjoyment payed off with a high mark.
Basically, I totally geeked out with my T&S and spent more time on it that most other people; I had a 300 item bibliography (though about 100 of them were book reviews and 100 journal articles) and some primary sources- I'd been to the county archives to look back through parish records, as well as looking at some other useful collections.
Ideally, you want to be at a point at the end of the holidays where you could sit down and write the essay- you don't want to give yourself too much to do over Christmas! I'd look through the books you've used thus far, look through Jstor, and generally see. Maybe start to jot down a few ideas- perhaps talk to people about the topic (which helps marshal your ideas!) reply to relevant threads on TSR (and, yes, I did like pwning sixth-formers with excessive amounts of detail...) or- and this is most fun!- find a whiteboard and some pens and do a big spider diagram. Start to think about where you want to go with your answer, so that'll help you get focus overall.
Hope that helps!