Your body is going into ketosis and will soon be burning fat for fuel. It will be worth it!! Keep the water content up and spread it through the day - at least 4 litres.
I found it took 2 or 3 days - but i was addicted to diet coke so i had major withdrawal from that but like mikey says drink plenty of water as it helps!
Also check out 4head - it's great and stops you having to pop tablets.
Good luck!