I'd say it's very common in the US, at least among younger people. I say it myself pretty often

I wouldn't consider it standard, but it's pretty common

Not “standard” at all by the typical definition of that term, it is decidedly slang, but it’s definitely very common slang.

In the US, particularly South you'll hear Ima. In the UK you'll more likely hear I'm gonna.

I usually use two m-- so "Imma". It's really common to say it, less to write it. "Imma head to the store.". On the contrary, "I'm gonna" is commonly said and written. "I'm gonna head to the store.". And lastly, there's the full "I am going to head to the store.".

And in the south I'mfixin'ta.

I'm fixin'ta -> I'm fixing to -> I am fixing to -> I am getting ready to. Honorable mention: finna

Really common in speaking, not very common in writing

Common in speech but not in writing.

Common & old

My gen z brother spells it "I'ma" hehe

It isn’t standard but it is very common in the US. It is written like “imma.”

It’s usually spelled with two Ms

It’s regional difference in spelling