Plant leaves may turn yellow for a number of reasons, such as nutrient deficiencies, pests, insufficient water or a lack of sunlight. A plant's leaves may also turn yellow as part of the normal growth process.
If a plant has yellow leaves due to too little water, watering the plant and leaving it in a dish to catch the excess water may help. Insects can deplete a plant of vital nutrients, and this may also cause yellowing. Root damage may cause yellowing leaves but may be remedied by adding fertilizer or repotting the plant.
Depending on what is wrong with the plant, improvement can be seen in as little as a couple of days, but in some cases, the plant will show signs of improvement within a couple of weeks.