I agree janxspirit. Sometimes my meditations lasts for 10 minutes and others for 2 hours. I find that paying attention to time can be a distraction. When it's time to change posture you'll know because your daydreaming will increase.
Since you're a novice though demonologist, you'll be daydreaming most of the time so just hold it until it is no longer comfortable.
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Sticks n stones'll break my bones, but if I land the first one, you're in trouble!