It’s a question I see all the time on Hacker News forums. Whenever someone posts a Show HN site that is implemented in React/Angular/Vue, this question shows up near the top of the comments.
It’s hard to say. Every time I’m on support, this comes up. So across the team, I’d say someone is dealing with these issues on any given day. We end up tracking things down by a variety of (largely indirect) means. So the support burden is probably a matter of 4–8 man-hrs per week over a team of 3 developers.