When to feed a plant isn’t a direct science and is really down to a gardener’s instincts backed up by a bit of basic knowledge. Container grown plants benefit from being repotted in fresh compost every year. However, even fresh potting compost has a limited supply of nutrients - six weeks supply on average, so after that your plant could start to lose vigour. Plants need a mix of nutrients, some in only tiny quantities, so care has to be taken not to overfeed. Garden centres offer a wide range of plant foods specially tailored for different plants from promoting prolific flowering or luxuriant greenery. You will also find specific feeds for geraniums, orchids, acid loving plants, etc but a basic all round fertiliser will be sufficient for most plant’s needs. How much feed depends on a lot of variables, how big the pot, how greedy the plant so the best course of action is to start off giving little and often and monitor the results.