I think you should be asking your wife this. For me the right time is every once in a blue moon, it better only be from the side and it better not be too hard.

Stormy has the right idea; if anyone knows the answer to your question, it's your wife. That said, if you don't want to talk to her about it directly or she won't answer directly, try pulling her hair lightly at some point while you're having sex. If her behavior, words, and sounds are encouraging, keep going; if not, stop (obviously).

If you're combining this with any sort of power exchange play and you're topping, I've found that grabbing the bottom's hair and forcing them to look you in the eyes (or, I suppose, to avert their eyes, depending on your preferences) while you give them an order can be particularly effective. The one other great opportunity for hair-grabbing that comes to mind is during doggy style sex; grabbing her hair and pulling her head back (slightly and do it slowly; you don't want to pull her hair out or give her whiplash) can be really hot for both parties involved.

Yeah you gotta ask your wife about this. For me it's never cause it really hurts when my hair is pulled and my hair falls out easily.

Huge fan of hair pulling just ease into it hun ask her also when it starts to get really intense rough type of sex that's prob when she wants it but sure if you ask she will answer this for you

Make sure you have a clear idea of when to pull, see above about talking. For the how, I'd recommend pulling closer to the scalp, it tends to give a more firm sensation without a lot of pain

I agree with others in that it needs to be close to the scalp, the key here is to distribute the pain. My partners need to grab a larger handful in the back of my head, lower than where my ear it. It's about controlling the direction that my face is pointing. It also needs to go with something, i.e. grab a big handful of hair and turn her head so that you can whisper in her ear or kiss her or what have you.