Question: When do I start feeding my fuchsias?
Answer: Fuchsias are gross feeders. The food in the initial fresh compost will last for about six weeks, depending on frequency of watering and type of compost. (multi purpose compost will not have as much added nutrients as a high quality potting compost has.) After this supplementary feeding should start if plants are not potted up into the next size pot and therefor fresh compost.

Question: What fertiliser should I use?
Answer: For general fuchsia growing use a balanced feed. Liquid feeds are the best as feeding can be carried out as part of your general watering. Follow the instructions on the packets and feed at full strength every 7-10 days or alternatively (and the preferred way with many experienced fuchsia growers) use ¼ strength and use every watering - the policy of a little but often. Frequent weak feeds are better than infrequent strong ones. It is just as with us humans we too prefer three meals a day and a nice variety in our diet rather than once large plateful of chips once a week. However, don't overdose - if it says a level teaspoon, don't give a  heaped one, if it says a teaspoon, don't use a tablespoon!
Proprietary feeds have their content analysis printed on the packaging. It is stated by the letters NPK and a percentage figure. The letters NPK stand for Nitrogen (N), Phosphates (P) and Potash (K). A high nitrogen feed i.e. has a NPK stated value of 30-15-15 the ratio of nitrogen to phosphates and potash being 2:1:1, a high potash feed i.e. has a NPK of 15-15-30 for a 1:1:2 ratio and on a balanced feed the three major nutrients in equal proportion i.e. 20-20-20.  Don't be confused if you see different figures quoted by different manufactures. 20-20-20 doesn't mean it is four times as strong as a 5-5-5 feed. It is just a different way to indicate that it is a balanced feed containing equal part of Nitrogen, Phosphates and Potash. 
Nitrogen (N) aids production of vegative growth (foliage and stems), Phosphates (P) stimulates the formation of the root system and Potash (P) encourages flowering and fruiting and also improves the colour, size and quantities of blooms. (Small quantities of trace elements are also present in the balanced feeds, Boron, Copper, Iron, Magnesium, Molybdenum & Zinc.)
Though good results can be obtained by applying the balanced feed throughout the growing season, it is more usual to use the high nitrogen feed early in the growing season to promote quick growth, use balance feed in mid season and switch to high potash once buds are forming.
However, too much nitrogen (especially in shady positions) can cause plants to make over-large leaves and soft leggy growth, susceptible to aphids and fungal attacks and not capable of supporting the blooms. Excess potash will lead to small leaves. Also, during a hot summer fuchsias will not need extra potash as they derive this from sunlight. An occasional feed of high nitrogen will then be beneficial to discourage them becoming prematurely woody
In very hot summers (which does happen occasionally in Britain) if it becomes necessarily to water your pot plants every day, even twice a day use plain water every other day to avoid overdosing. 
Finally beware. Plants must not be bone dry when feeding. If dried out water first before feeding.

    Also a Society Quiz , thanks to Joe Adams.