As a general rule, hedges that are pruned more regularly will usually require more fertilising.
The majority of hedges will only require fertilising once or twice per year, usually in spring and late summer. But this will depend on the recommended fertiliser requirements of the plant you have chosen for your hedge.
When it comes to what type of fertiliser to use for you hedge a complete organic or slow release fertiliser is recommended for best results, but there are exceptions. For example if you have a hedge that requires more regular pruning you may want to consider a fertiliser that is higher in nitrogen, such as a lawn fertiliser or pelletised manure fertiliser, these types of fertilisers help promote new growth. For natives, it is generally recommended to use a fertiliser which is appropriate for use on native plants. Whichever fertiliser you use, it should always be watered in well.
As for screening plants, these plants tend to require far these pruning than hedges, and as a result should only need fertilising once a year at most, and even that may not be required. This will again depend on the type of plant you have chosen for your screen, most plant labels will contain details on the frequency you will need to fertilise.
Fertiliser should only be applied when the plant is not water stressed. Also try to avoid over fertilising your hedge, this can often lead to faster but ultimately weaker growth, meaning the hedge will require more pruning, and this can also lead to significant issues with pests.