How long should H2 tags be?

A tag defines the second-level headings in your webpage. So let's wrap up on H2 tags: H2 subheading tags get used to break up content and make it scannable and easy to read. H2s are used to highlight important pieces of information and head up subsections on your page. We recommend sticking to a maximum length of 60 characters

A H2 tag length should be just as long as your information , like if you writing for a SEO plugin, your heading can be like
Why we use SEO plugins, or top free SEO plugins or top free SEO plugins to use.
As, h2 is content based and not includes in title/meta description, there is no word limit.
But you should keep around 40-50 in length or just as much as it fits your content.