How much impact does it take to get a bruised periodontal ligament?

Not just how much. Can also be what you bit on or what hit your tooth. Doesn't matter much, because the bruised ligament will heal very quickly if you don't challenge it for few days - chew soft food and not on that tooth. Treat it like a slightly sprained ankle and you'll do just fine.
Quite a bit. You can bite with a lot of force so to bruise one would be difficult under normal function, but if you unexpectadly hit it with a bone or something very hard that you shouldn't bite on, that will do it. A bruised ligament is really tears of some of the fibers and that can make it very sore to bite on and even feel swollen, not look it though.