The duration of your traffic management (almost certainly 5 hours, but consult the table in the linked document) begins from the moment you exceed the prescribed value, and continues for the appropriate time, even if this takes you past the management times. For example, if traffic management kicks in at 20:59, it will continue until 01:59.
There is an additional and separate policy for peer to peer traffic (such as Bittorrent downloads) and Newsgroups downloads. In this case, a speed reduction is applied during the peak times, regardless of the amount you have downloaded.
Hope this helps,
I don't work for Virgin Media. Any advice I offer is correct to the best of my knowledge, but is taken at your own risk.
The download are unlimited other providers charge if you go over your download limit Virgin just limits the speed for those who download heavily during peak times not to affect other users on the network.
All views expressed by me on this forum are my own, and do not in any way reflect the views of Virgin Media.