0% I'd get very suspicious if it were to start working in the first attempt.

P.S.: I code in JavaScript

How many tries it takes to get code to work is a good measure of 1) the complexity of the problem and 2) how skilled you are at that language.

The more experience you have, the more you collect good habits which prevent doing something more than once. "If I do thing W it's likely to break thin X", or "Always do thing Y before thing Z," that kind of stuff.

Assuming that the input and output is defined the answer is roughly 100%. Usually I break it afterwards by refactoring and cleaning it up.

but the PoC usually always works on the first attempt. But I guess you would have to add 'in a language your proficient' because I guess my rates would plummet if I would write such things in a language I don't know or understand.