People beginning to practice Nichiren Buddhism generally start by chanting Nam-myoho-renge-kyo (also known as "daimoku") for a few minutes, morning and evening if possible.
Buddhism is practical, and beginners are often encouraged to try chanting for a certain period of time, to get a feel for the practice and see what changes they notice, such as increased hope or energy, or improved relations with others. It is helpful to have support from a friend, or a local SGI group, in order to find answers to questions that arise.
How long to chant for is a matter of personal choice. When people first start to practice often they begin by chanting for 5 or 10 minutes at a time. The most important thing is to try to make this a regular part of one's daily routine, if possible, morning and evening. Continuing is a constant challenge but one that reaps positive rewards. Often when facing obstacles, people will chant in a fully focused way in order to see themselves and their situation clearly.
Chanting has been likened to charging a battery, so the more highly-charged it is the more energy one has to expend. It is important to remember however that chanting is not magic. It fills us with hope, strength and energy so that we can take the right action to resolve our problems. It is natural to chant for people we know who are suffering from illness or other problems in life.