Start with 10 minutes and only sit longer if you feel the length is too short. Don't force yourself to meditate longer if you are not ready to do that. In time you might like to extend your meditation to 25 minutes. That's a length that allows you to settle your mind without causing too much stress on your body. Most importantly, shrug off any 'shoulds'. Some people enjoy sitting for an hour at a time. Others find that they can't sit longer than 10 minutes. Do what feels right for you!
