Oleander is a tough drought resistant shrub so I think the size of pot it has will be fine for a number of years. Of course most plants are happier and will grow larger planted into the ground and a lager pot would allow the plant to go for twice as long without needing repotting. Once you see the plant begin to decline in health and the leaves appear dull or yellow you should consider repotting into a larger container. Keep growing, Marianne Binetti