Databases should be able to scale for millions of records, so the amount should not be the problem.
So what are you looking at?
A Discussion Part
A Response Part
A discussion contains a collection of responses
A Response is attached to a User Part
I think this should be fine. This will also allow you to do stuff like show history etc...
Most big systems contain clean up processes, so you would have a class that implements IBackgroundWorkerProcess (i think its called in Orchard) to archive off all responses that are '30' days old for example.
Hope this helps.