It will be either $600 or $0.
I made too much to qualify in 2019, but not in 2018. I filed an extension in April and filed in Oct. The IRS sent a notice last week that I need to confirm the return is actually from me before they will process, with a deadline of Jan 8th. So, I'm hoping I will get the stimulus just as I got the first one, based off my 2018 income since they haven't processed my 2019 return, yet.
If I get it, it's not enough to make much of difference in my spending. So, I'll try to stimulate the economy with it, eat at some restaurants after I've had my second dose of the vaccine, and donate more to the local food pantry.
Edit: After looking into it further, I found that they are not using 2018 returns for people whose 2019 have not been processed, which differs from the first stimulus. And I won't be able to claim anything on my 2020 return, either. So I won't be getting anything. My 2019 and 2020 AGI's are both above the $0 phase out.