I'm doing 10 subjects, not 11, but I'm sure the same things apply. I'm doing GCSEs this year too, and I'm not stressed, but worried that I won't revise everything in time.
That aside, I'd recommend you to have 25-30 minute sessions, then take a 5-10 minute break. Repeat this for each hour you do, so eg 50 minutes studying and 10 minutes resting. Splitting it into sessions like this makes it easier for you to retain information.
But retaining it for the long-term? Use spaced repetition - repeat the information every so often, eg every two days, then every week, then every two weeks, until you get to the exams.