Start with an audit of your weekly time commitments, keep in mind school activities, social activities, and any other regular requirements you need to spend time on. This will leave you with a good indication of how much time (and when) you can spend building your business.
Keep in mind time usually equals money. The more time you can dedicate to marketing your business, the more successful you are likely to be. So if you find you are limited in time, be realistic about your goals, or ensure you spend your time in the most efficient way possible.
If you are very organised and have planned well ahead you can achieve great results in an hour a day!