8 weeks should be long enough, unless you have been bordering on being an alcoholic when you may need 12 weeks.
Half life is simply the time that an item takes to loose half of its content or radioactivity. In the case of CDT, which is what is tested at the DVLA medical it works like this: let's say you have a high CDT reading of 8%. The half life of CDT is variously reported as being 14-21 says. Let's say 21 days, so after 21 days the result would have 'decayed' to 4% - halved. After another 21 days, this would have decayed to 2% - halved again, and so on.
I mentioned radioactivity ..... this is an easy half life to predict. Whey you are talking about something in the human body then thinks do not always work in such a clockwork way so half lives are best estimates and cannot be taken as gospel, just a good indicator.
Hope this helps.