How long is h1 supposed to be for SEO?

Hello. The headline H1 is one of the main headings in SEO. I know that this headline should contain the main key phrase. The keyword I always use. But I have doubts about the length of the header. I always write H1 on one line and I think that the headline shouldn't be very long such as the Description meta tag. I try to keep the H1 headline short and clear, but I think there are exceptions. I want to know what H1 header length is recommended and what maximum H1 header length should be for SEO?