Pot-grown plants are like caged animals; being confined, they can't 'forage' so they rely totally on us for food. Most compost only supplies 'starter' nutrients that are soon used up, so begin feeding plants growing in pots six weeks after potting or repotting.
The amount of feed plants need varies according to growing conditions, size and speed of growth. Some people underfeed, while others will overdo it. If you apply a strong dose of feed all in one go, the resulting strong solution of salts can actually draw moisture out of the plant by a process known as reverse osmosis so over-feeding does more harm than good.
Always follow the feed supplier's advice, but as a general rule start feeding in spring, perhaps once every two weeks. Feed weekly when plants are growing vigorously and the weather is warmer, rising to twice weekly for heavy feeders or fast-growing plants that are in large containers.