In a period of time

Question

What's the best way to express in in the following sentence:

I will go in five minutes time.

Here's my attempt　　　　

五分かいあとで行きます。

Here's another attempt which I have even less confidence in:

いまから五分行きます

Assuming the above are correct then I would translate them as "I will go after 5 minutes", and "I will go 5 minutes from now". So, questions:

1. Are the above correct?
2. If not then please explain what I've done wrong.
3. Are there better ways to say it?

Please excuse the inconsistent kanji. I've only used the kanji I already know.

Answer 1

The easiest and most commonly-used structure for expressing "come/go/return, etc. + in + (time period) " would be to use particle 「で」 and say:

「(time period) + で + [来]{き}ます/[行]{い}きます/[戻]{もど}ります, etc.」

This would by far be the most versatile way of expressing "in (a time period)"

Other expressions:

「(time period) + [後]{ご}に + (verb phrase)」

This expression is less versatile than the one using 「で」 because it cannot be used to make a general statement such as "Instant ramen cooks in 3 minutes."

「インスタントラーメンは3分でできる。」 can be said anytime as a general statement because it is "true".

「インスタントラーメンは3分後にできる。」, however, can only be said when you are actually starting to cook instant ramen.

I have seen quite a few Japanese-learners make mistakes in using these two expressions correctly.

You can also choose to place 「あと」 in front of the time period and say:

「あと５分で行きます。」

Answer 2

In your first translation, I believe you used かい as translation for English "time".
But かい(kanji 回) means time as in the first time, many times. Equivalent to German "Mal".

So for the first one, let's drop the かい.

5分あとで行きます。

This is a fairly understandable sentence, but not natural enough.
When expressing in five minutes, it's more usual to say 5分ご than 5分あと,
although when written in kanji the both are the same, 5分後("go hun go" or "go hun ato").

We're trying to indicate the time on which the speaker "will go", so we should use the particle に instead of で.

5分後に行きます。

This sentence sounds natural enough.

Your second translation was actually a little better.
What it lacks is, 後に.
We use
いまから5分後 : five minutes later from now
に : at (the time)
行きます : (will) go
So

いまから5分後に行きます。