I didn't know there is such a flower known as "Forget Me Not Flower" . LOL . Yes , You can swap your materials but you can make the final Call .It all depends on the significance on the material to you .
As for how long those the spell to kick in , It really depends on whether you have put in enough energy into your spell and how successful your casting was. Depending on how your spell was done , The time taken can Vary.
Most of my spells take 3-4 days to manifest and some even a week or two.
Perhaps you should just be patient and just wait for your results to occur. If you cast a spell Properly , it will definately manifest