I have a friend of mine who has his mother plants for more than 2 years and he has told me that the plants can last even more, as long as you give them a proper care. What i know from him is that these plants need to be in the vegetative phase the whole time, and for that to happen they need more than 12 hours of light every 24 hours. Every day! Also, he uses fabric pots for the plants, in order to preserve the roots for longer... the trimming process is veery important too.. just try and test what works for your plants.