I agree with @MichaelLiebman 's comment, it depends ...
My end product usually has about 30% of BRoll, I.E. every 7 or so seconds I'll add some BRoll.
It's possible to adjust your percentage 20-30% either way. The way you'd have 60% broll and still call it B-Roll is that the A-Roll had a lot of failed shots and it was broll that saved the day.
As a rule seven seconds is short and > than 20 but < than 40 is probably good, but even 7 can be long (parkour, skateboarding, etc.) and 40 short (rowing to the crab traps, driving over the Golden Gate); it depends.
So basically what does it depend upon (limiting it somewhat to the scope of your question):
Subject and pace of the story (laidback vs. jarring).
The need to switch. If you must show wide and close with fast pace you'll be flipping back and forth in less than 7 seconds (showing what someone is doing or offering how-to advice), if you're panning around someone's surroundings or showing the parts or usage of something you can run well beyond 40 seconds.
What you got. You either have to make an edit with the shots you have and try to cobble them into your storyline or you need to shoot some more footage (which often, but not always, involves going back out to the location).
Style. It's yours, make it your's and be proud; assuming it's reasonably good.
Look at what other people do, read up on the subject; but make your own style.
Videos are easier than written advice for this topic but here's a mix of both:
Nikon's "Shooting B-Roll Footage with a DSLR". In particular see near the end of the video included on that webpage how B-Roll saved Grandpa.
Look at this video on the Gear Dads YouTube channel: "How To Shoot B-Roll". See what and how to shoot, so you come home with useful material.
Speaking of YouTube, there's lots of professional advice, or simply work done by professionals (that isn't explained, but you can see how it's done). Erik has been a Producer for over 2 decades, here he is interviewing at a Trade Show: "CAME-TV Gimbals – Newsshooter at IBC 2017". Single camera, single person crew, and he edits it too. Not perfect, not bad. One man job.
Look and learn, practice makes perfect. There's also short courses at Colleges or offered by local talent that can provide individualized advice.
If you edit your question and put a link to an example of your work then fine tuned advice can be offered.