IRS says to allow 21 days for issuance of a refund (paper checks will then have to go thru the mail). But they also say that some returns may take longer than others. So that's their way of saying they can't guarantee any specific timeline.
This year, in particular, they are holding refunds for taxpayers claiming the Earned Income Credit or Additional Child Tax Credit until February 15.
Here's a link to the IRS web page where they discuss this issue: https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions