"There are two different views on this: 1. Any time spent meditating is good, no matter how short. 2. The longer you meditate for, the better. The thing is, they’re both right. Even five minutes a day is better than nothing, and a longer meditation session can be more beneficial than a shorter one.
Half an hour a day might be a good starting point, but the key is what works for you. The most important thing is keeping a regular meditation practice. If you practice for so long that it infringes on the rest of your life and stresses you out, or it feels like a chore and you don’t enjoy it, then you’ll be less likely to do it every day.
There is one objective guideline, though. You should meditate long enough for your mind and body to settle down. Only when your thoughts have calmed down and your body is relaxed can you really start your practice. That may take the first five or ten minutes of your meditation session, so try to sit long enough to go through the settling-down phase and have enough time left to really enjoy and benefit from the practice."