×ここに来ていつですか is incorrect.
So in English you have two different ways to convey this type of information:
- I came here 3 months ago.
- I've been here for 3 months.
And there are two different ways to ask about it:
- When did you come here?
- How long have you been here?
Note that you have to choose the correct interrogative. "When" obviously does not work in Sentence 2. (×"When have you been here?" is ungrammatical.) And this is exactly why ここに来ていつですか is an incorrect Japanese sentence.
Apparently, you already know there are also two ways to say this in Japanese:
(に can be dropped in casual speech)
(literally, "I came here and it's been three months.")
As you know, turning them into questions is a simple process in Japanese. Just replace the part being asked with an appropriate interrogative, and add か at the end of the sentence:
But note that いつ does not work in the second sentence! It's because "when" and "how long" are two different types of questions. To ask about a time span (i.e., "how long") in Japanese, you have to use どのくらい or どれくらい, not いつ. Alternatively, you can use 何か月 ("how many months"), 何年 ("how many years"), 何時間 ("how many hours") and so on.
The difference between the two is small, but the latter using どのくらい would sound more natural when you focus on the experience during the period.