How bad does a dirty barrel affect accuracy?
I can't answer that question, because I've never let a barrel go to the point of inaccuracy due to not cleaning it.
When shooting centerfire benchrest in the early 1970's there was a rule of thumb about cleaning barrels, and it was shoot no more than 20 rounds and clean the barrel. During a relay, we'd shoot "maybe" 20 rounds max through a barrel. This counted sighter shots and record shots, so it worked out well from that point of view. We all cleaned barrels after each relay.
I still use that rule of thumb, but that doesn't answer your question. I don't personally know who could honestly answer that question as there are far too many variables to consider. I think any answer would be a generalized reply.