Technically schools do have a firm deadline because of the May 1 National College Decision Day. If you look at college websites, they often say decisions will be out by April 1. For instance, USC says:
Applicants selecting or deferring to Regular Decision will be notified of a decision by April 1, 2023.
But they typically release decisions on March 24:
Based on previous years (decades actually) March 24th has been a favorite for them. So that would be my guess, but they are in charge!
It’s sorta like how students can submit their applications early. Colleges can send out decisions early too. 
I know it’s not much of a consolation, though. Waiting is stressful in the best of circumstances and the uncertainty makes things worse. My son applied to Cal Poly SLO for transfer, but hadn’t been all that interested until he visited last week. Now he’s suddenly checking the student portal compulsively. 
It’s part of life. We have to wait for potential employers to make decisions too. In my experience, that can be anywhere from a few days to a few months. Not saying we have to like it, of course.