Usually straight after, but it can at any time. Shaving before it grows out won't really do anything. To lessen the itching use a new razor each time, plenty of shaving foam and soak in water for ages. Only shave down-wards untill there is practically no hairs then you can go upwards.
After that use a moisturising lotion, you can get some for sensitive skin or itch releif and apply it whenever its itchy. Wear loose fitting clothes.
Hair grows constantly. And, once you start shaving the pubic hair, you could expect to see and feel new growth stubble within a day or two. But, you could start feeling the itch almost immediately after shaving... depending on how you shave. That is to say... the technique and the kind of device used to shave with.
If a razor is used, the shave is closer to the skin... but along with that there's a greater potential to promote a rash/ itch due to the likelihood of scraping and irritating the surface of the skin (razor burn). If you use an electric shaver, you won't have quite as close of a shave... and, there's less potential for rash/ itch and cuts. But, you might get something of an irritation as sometimes shavers tend to tug and pull on the hairs.
So, in answer to your question, you could feel itch right away after shaving. But, once you get used to shaving, that itchiness tends to be less frequent. I think that's because through practice one develops skill and is able to minimize the potential to create irritation. Plus, I think that the surface of the skin becomes a bit less sensitive and more accustomed to being shaved.
And, as for keeping it shaved to prevent itch... unfortunately, frequency in shaving isn't a guarantee to prevent itch. From my experience, it can actually promote more itch/ irritation. But, again, I really think that it all comes down to developing technique with this. The more you shave, the better skilled you'll become with it... and you'll lessen your likelihood of having to deal with rash/ itch and cuts.
A quick answer...I use men's after shaving lotion from Victoria's Secret!
This not only prevents the itch BUT.....also prevents ALL razor bumps!!! True Story!!
I find that if I shave every two to three days, I minimize the itching. but I have also been shaving for a while, and the after shave idea, excelent! I have never thought of that, feeling good and smelling good in that area is very important to men and I happen to think its wonderful when a man is clean shaved as well, very sexy.