The length of meditation is dictated by how you feel. If you are benefiting from meditation you will sit longer. If you are disturbed, however, it is a good idea to start with the Heartfulness Cleaning for 10 minutes, and then attempt meditation. If you are able to sink into deep meditation within a few minutes, then perhaps you only need to meditate for a short while. If it takes longer then sit for a longer period. There is no need to attempt to go into a state of deep absorption or Samadhi. Let it happen naturally without force. On some days you will achieve greater depth than others.
Initially, try to meditate for at least 20 minutes daily, consistently. Then you may gradually increase the time to an hour as you find your rhythm. The only strict guidance is that meditation should not exceed one hour in duration. Meditating for too long creates mental pressure. If you have the time and inclination to meditate a second time, however, please do so, provided that you give a gap of at least fifteen minutes in between. There is no restriction on how many times a day you can meditate, as long as you do not neglect your duties.
Ultimately, our lifestyle becomes meditative. No matter what activity we are engaged in, we remain meditative and centered within. The act of meditation triggers an ongoing meditative state, which supports all our activities.