In order to maintain your range of motion, a weekly stretch session is sufficient. However, if your goal is to increase your flexibility, you need to stretch three to five times per week, and you need to be consistent. It may take several months for certain stretches to become comfortable, but perseverance is key (just make sure you are not pushing yourself to the point of pain.) Make sure that you feel soft burning feeling during stretching.
As it takes time for your muscles to adapt and adjust, you must give them time to heal, rest and repair themselves. This means mixing up your stretching program by alternating light days, heavy days and rest days. Any gym bunny knows the importance of working and resting different muscle groups on different days, and making gains in flexibility follows the same principles.
Though it may be tempting, over-stretching increases the risk of injury and can just push your goals even further away. Always mix stretching with strengthening exercises together to keep your muscles safe and injury free.
Stretches should be slow and gentle, and should never create a sharp or painful feeling. Use your breath to assist you, and do not bounce!