QC stands for Quality Control / Check, as such it is the last phase of the project. What's done in the QC phase is:
1) reading through the translated lines for typos,
2) checking that the translation is consistent for each route (e.g. the way the character speaks is constant),
3) line placement is correct (i.e. the words fit in the text box - this rarely needs fixing, however),
4) fixing the above errors if found.
Assuming the QC'er is a native English speaker this phase doesn't take that long. At most it will take twice the amount of time it takes to play through the game once - if there are no typos and the translation is consistent, it takes exactly the same time as playing through the game once.
Thus the answer to your question, Apollo, is: a 5000 line QC takes less than 7 months. Depending on the reader it takes less than 7 hours.