You may have noticed many gyms and PT's sell 12 week transformation camps. That's because that is typically how long most people need to see some real change.
Yes, some will offer an 8 week transofrmation, but that may not do it. :S
Depending on where you are now and what your goals are - on average it's going to take around 3 months to get to where you hope to be.
Christmas is coming and that means you should already be working towards your goals.... Need a Personal Trainer to get you results, then get in touch.
The first line asked a question.
The real answer to the question "When should I start exercising" is always: NOW!
(Seriously, why would you put off improving your health and fitness?)