how severly can you prune the dwarf willow and when is the best time?

Dwarf willow in Britain means Salix herbacea, which is not a tree but a very small ground covering willow for a rock garden. I assume you're not talking about that because you speak of "trees", so although I'm not sure which ones you have, the general advice with willows (except the ones grown for their coloured bark, which are kept stooled) is to prune as little as possible, only what's essential. Cutting a lot will stimulate excessive growth - if you want to prune, do it when the trees are dormant, during winter.