You will have your speeds reduced for 5 hours from the time you go over the limit.
In theory you could go over your limit at 8:59pm and lose your speed until 1:59am.
Full details of all the Thresholds for each BB tier, the times they come into force and the duration of the throttling can be found
here.
Also note that your downloading is restricted from 10am to 3pm and 4pm to 9pm with an additional restriction on your uploading from 3pm to 8pm.