I would say every 2 weeks give the allotment a good feeding. Fruits and vegetables are very hungry plants. What to use? It's not an exact science. The market is flooded with all kinds of options. I think organic is the best option - good rich compost to start off with. I use Alaska Fish Fertilizer and Doctor Earth organic Fruit and Vegetable Fertilizer. If you can find something similar, you'd be in good shape.

I usually say once every two weeks, if you are using a liquid, or once a month with an organic granule--like the Dr. Earth. I eschew pure chemical fertilizers these days, since the organics are better for the soil structure, and the beneficial organisms there. If in doubt about the amount to use, err on the side of caution. A little too little feeding just slows things down a bit, while too much can kill the plants. If the soil is well maintained with compost, including compost made from the inedible parts of the plants that were grown in it, it will need less feeding from outside sources.
Leafy veggies, like lettuce, need a bit more nitrogen. Veggies grown for flowers, fruit, or seed need a bit more phosphate, especially in acidic soils. Root veggies like a bit more potash Balanced fertilizers are a good compromise "shotgun treatment".

Bear in mind that if you use a granular fertilizer and the ground remains dry It won't be able to reach the roots. If your soil is in good heart to start with and has lots of good organic stuff early in the season I honestly wouldn't worry too much about feeding after that.

When we grew veg on our good rich soil we rarely resorted to feeding the plants, they did not need it. Too much food can result in overgrown 'watery; plants with little or no taste. So, only feed if your plants are not growing well.

Thank you for your advice.