When you start using affirmations you will need both. I have used affirmations that have started to work within a few weeks. Others I am still working on months later. It depends on the nature of your goal. It depends whether you are trying to turn around negative thinking.
Negative thoughts are sticky.
Be prepared to practise your new thoughts just as devotedly. Believe me, the payoff will be great and your efforts will be rewarded. While it would be nice to repeat affirmations a few times and have them come true with the tap of a magic wand, it just doesn't happen that way. You have to do the work that moves you toward your goal. But you won't get anywhere with it if you don't do the work.