It depends. If all I want is some prototype or pet code project, my LLM can write most by itself. The speedup could be 10 times or more. However, if I'd let a LLM write code for my work, I'd have to very thoroughly review it and most likely ask it to rewrite it several times. Each time this would require a new review of course. There would still be a speed up but I guess at most somewhere around 25%.

In practice I try to combine the best of both worlds. I write some code by myself and rely on my LLM for parts that are not too big and where I expect it to do a pretty good job.