Depends which one you're taking
If you're taking Paludrine and Alvoclor then you need to start taking them one week before you go. (Paludrine and Alvoclor are the one's where you take two large tablets and two small ones on the first day of each week and then two small ones every day after that. You need to take them one week before you go, all the time you are there and for four weeks when you return to the UK.