when do i start to feed my cucembers

The usual advice with most vegetables like this is to feed once the fruit have begun to form (tomatoes, peppers, cucumbers etc.) If you give too much nitrogen before this, you tend to get more leaf than fruit.
I would use a proprietary tomato feed as this is high in potash. Alternatively use something you make yourself (which is a lot cheaper and just as effective) such as a liquid made from soaking comfrey in water, or nettles. Even better (and smellier) is to get some farmyard manure, put it in a sack, and soak this in water. The liquid feeds from these kinds of things are good for all these kinds of crops.
I would feed two or three times a week, depending on how well the plants are growing. It also depends whether your cucumbers are in the soil or in pots. Pots tend not only to dry out more frequently but are also limited in nutrients.