When it's got the moves you want it to learn. Nidoking barely learns anything on its own, so you have to have it learn moves as a Nidorino. Or are you asking for moveset help? --- Daughter, to her brother: "I find your existence infuriating."
Nidorino doesn't really learn anything good. Well, you have Horn Attack, which is workable ingame but not at all spectacular, and Horn Drill, which is way too inaccurate and comes way too late (level 53). Oh, and Helping Hand, but I don't think that move sees much use even in competitive 2v2.
The best time to evolve Nidorino is as soon as possible, unless you want to use that Moon Stone on something else. --- [Trolling] includes [...] using another users quote in your signature without permission. ~ http://www.gamefaqs.com/features/tou.html
I would say after Horn attack, this is what i did. Nothing really special enough to keep him a nidorino after that. --- "that's how I KNOW you're a redneck. Clearly you've described a Lumberjack, and only a redneck could make that kind of mistake"-Dom0fMHF