How much does one read until it's enough?

Milkman

Do you have to read all of Irenaeus's writings to fully understand his thoughts on a particular doctrine? Say, his recapitulation theory?

This goes as well for any of the other 'ancient' writers.

Do you have to read, for example, all of Origen to understand his views on the 'ransom theory'?

Or, do you pick and choose those primary texts and leave out the rest of their writings?

mm. 

MJ. Smith

No, one doesn't need to read everything. But the amount you need to read depends on the author, the topic, and the depth of understanding you need. Sometimes I read as little as the context from which a particular quote is drawn. Sometimes I read a couple of books and a study of them.