It’s supposed to given at everyone’s discretion when and when you think someone deserves. It can be for help they’ve given you or others, an informative post or just because you think they deserve it.
We have had a few people abusing the system of late and giving people negative karma without reason. We are monitoring those people (albeit a pain in the arse to do) and they well be dealt with if it keeps happening.
The other thing that is curtious (not mandatory) to do is to tell people when you award of deduct karma 
Happy karma’ing people 