I think it's generally best to take your tablet as close to your meal as possible. The idea being that Xenical only works on the food you're eating at that particular mealtime.
If you forget to take your tablet straight away, you can take it up to about an hour after your meal, but there's not much point after that, as your food will be well on its way to being digested.
Some people find it useful to take their tablet before or at the start of their meal as an added incentive to stay on track. You don't get so tempted to 'cheat' and have deserts/fatty foods when you've already taken your tablet, because it means you'll pay for it later on the loo!