I know there are some quirks, but I can't remember all the details atm. I would think that once your data is in there, you can get to it. But, I also think that records only go back so far, i.e., you can only run 6 months at a time. I'm not sure who to tag in to ask, so you may want to file a support case to be sure.
One way to get all replies is to go to user reports and just run one that covers all time. When you total the replies column, it'll give you that number. But if you have hundreds of thousands of users, it's going to take a while to run and it's going to be a really big spreadsheet. You can limit the columns, though, by removing what you don't need for that report. Then just sum up the appropriate column. Clunky, but it should work.
There may be a more elegant solution, though.
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