The recommended gap between mRNA (Pfizer ad Moderna) vaccines is 4 weeks. This is what the CDC has in its directives. This was the regimen used during the vaccine testing phases. It's the same across all states. There may be some variation across states based on the roll outs.
I think the UK change to an 8 week gap for AZ was based on trying to get a first shot out to as many as possible and not based on the clinical trials (which I think was 4 weeks).