The imagery does have a cumulative effect over time, but it varies from person to person how long it takes for it to take hold.
The more you practice with it, the better chance you have of increasing its impact, of course.
And sometimes it will feel like nothing's happening, and then one day you notice that you've been feeling less anxious - either the anxiety isn't as intense; or it's not happening as often; or both.
It's usually an organic kind of phenomenon, so it's likely to sneak up on you, rather than being a dramatic change you're likely to notice immediately (although that sometimes happens too.)
Hope this helps.
Best wishes to you.