I don’t know about “instantly,” but at times I’ve seen results (often of unwitting manifestation) within a day or two.
That’s why my roommates in Sedona used to call me “the manifestation queen”: I would think of something and put a lot of emotion into it, and voilà! it would pop up the next day.
The secret to that: practice, practice, practice.
At the time, I was so “in the flow” that I had to be careful what I was thinking about because it would manifest like a swarm of fruit flies on a banana peel.
For example, I loved cats but couldn’t have one because my roommate D., who was the lessee of the apartment, was scared for the lives of her two iguanas.
At some point, our next-door neighbors’ cat had a litter of kittens. The neighbors went away over the weekend and asked me to cat-sit. The little ones were so adorable that I found myself desperately longing to have a cat of my own.
The next day, I was reading Tarot cards at the Earth Mother Father Foundation when one of my clients, a tourist from Australia, walked up to me with a cat carrier in her hand that contained the cutest little kitten.
She said she and her daughter had found it near their vacation rental and fed it for a while, but now didn’t know what to do with it since they were going back home the next day.
She practically begged me to take it, and of course I caved.
When I got home, D. freaked out on me and gave me an ultimatum to get rid of the cat. She said it needed to be gone within the next week and until then had to stay in my room where it couldn’t "threaten" her iguanas. (Though considering the size of the little critter, the iguanas could have quite easily defended themselves.)
I realized I had manifested the kitten and now needed to “un-manifest” it. I didn’t want to bring it to a shelter, so I focused my intention on finding a good home for her.
Two days later, I went to my improv theater group, and one of the members was crying because she had lost her beloved cat. It had somehow gotten out of the house and had been missing for a few weeks already, but this was the first time she talked about it.
She said she had hoped the cat would show up again, but at this point it was likely that it had been eaten by coyotes (there were a lot of them around) and that she had given up hope.
She had never mentioned the missing cat before (or even that she owned a cat). I persuaded her to adopt the kitten, and as far as I know, they lived happily ever after.
Just one of many examples. So, yes, (almost) instant manifestation is possible. But remember, be careful what you wish for, and work with God instead of against him. "Thy will be done" should always be at the forefront of your mind.