If you can provide close to ideal conditions, and that is hard to do outside of a greenhouse, then a plant has an almost unlimited lifespan. However, the farther away from ideal conditions you are, then the more rapid will be the appearance of symptoms and decline. Just because you get a yellow leaf today does not mean it was due to something you did yesterday or even a week ago.
It is not commonly understood that plants use nutrients in extremely minute quantities, especially in less than ideal growing conditions. It is very rare for a plant to deplete all of its essential soil nutrients. Rather than repotting and risking disturbing the roots, it is better to add some dilute fertilizer occasionally if you have reason to believe that there is a nutrient deficiency.
Weak and spindly growth is most often a symptom of inadequate light and rarely due to lack of nutrients.
