anyone knows what is this limit in term of volume traded?
Nobody knows, nor is it stated clearly in their new terms and conditions. You could have a ban going on you at any moment, and you'd have no chance to argue against it, as long as they think that you're trading with "significant trade volume". It's a completely one sided judgment, if you know what I mean.
For me, the restrictions came onto my account when my trade volume was just around 2-3 BTC or so, over the lifetime of the account. No way was I even an active trader, but they still decided that my volume was significant.
And that's the thing, they should at least tell you when the requirement for verification will come. That way, people don't get a nasty surprise one day but can know in advance. But hey, everything in the EU is tighter now and probably is going to go in that direction in the future as well.