depends on how you are growing them, how you have prepared the soil, and at what stage the plants are at.
Well prepared soil should require no feeding from now until July. Time to give a boost is when the length of the stems starts to shorten and the number of flower buds drops from 4 to 3 if you are growing the spencer varieties.
Watering is far more important than feeding...a good soaking early morning a couple of times a week in dry spells should keep them growing nicely.
As to fertilisers, Chempak no8.... low nitrogen.... supplemented with liquid humus and liquid seaweed extract are what i use.

Which Lathryus is it though ? ?
I have Lathyrus latifolius and don't give it any special attention.