If you got the Amex card in July and received a CLI after 90 days, that would be October. 6 months from then is April; you can't get another Amex CLI until April so it is not worth trying in February.
Discover doesn't have any strict guidelines or time periods required for waiting and their CLIs are all very profile-specific. Some people get CLI's every other month, some every 3-6 months, others every 21-31 days. It's really all over the place so all you can do is make requests on occasion and try to determine what sort of request schedule works best for your specific account/file.
I believe creditors care about your utilization with them just as much as overall utilization as the higher your utilization with them, the greater risk they are at. I personally have never requested a CLI if I didn't have single-digit utilization on both the account I was going for the CLI on AND overall utilization. You always want to give yourself the best possible look with respect to the creditor in question, so the first thing they are going to look at is how you treat their card. If you have 9% utilization with them but some 30-50% utilization elsewhere, at least they see that you are treating your Discover account the best which I only think could be favorable for a CLI.
While I personally recommend being at single-digit utilization for CLIs, others have reported getting CLIs that are just as good when in the 10-29% range. Once you get to 30%+, though, you are looked at far less favorably and while you may still be able to receive a CLI there is no doubt in my mind that it will be watered down a good amount from what you could have gotten at single-digit utilization; It could be the difference of receiving just $500 instead of $2000, for example.
With Discover you don't really ask for a number like you do Amex. You simply request a CLI with Discover and they'll grant you whatever your profile tells them you deserve at that time. It could be nothing, $500, $3000, etc. At least with Discover you can request as often as you'd like. Assuming your utilization is in check (below 30%) I'd recommend hitting the button with Discover every month about 4-5 days after your account reports. Some months you won't get anything, others you will.