Much like a person, a plant's needs make themselves known by it's outward appearance.
Excuse my finger
After a certain time, a plant outgrows its environment. It becomes pot-bound. It's growth may be stunted, its leaves may become dull. These are the signs that it's time for a change.
A roomier pot, some new, rich soil in which to transplant will assure not only the plant's survival but it's blossoming. After all, what animal or vegetable wants only survival when thriving is an option?