Well it will depend upon your practice of meditating. If you're meditating from a very long time then you will easily be able to meditate for long. And m if you think you don't have much time for meditation just go for short sessions of meditation in a day. Beginners will not be able to concentrate more than 5 minutes. So starting with a small step you can meditate on daily basis whenever you feel overwhelmed. Otherwise Conclusion can be that we don't need to do meditation for a long time. Small steps can also make big changes in our lives.