The best I can tell you (from working with LLM's) is that... it's complicated.

There are moments where spending 10 min on a good prompt saves me 2hrs of typing and it finishes that in the time it takes me to go make myself a cup of coffee (~10 min) Those are the good moments.

Then there are moments where it's more like 30 min savings for 10 min of prompting. Those are still pretty good.

Then there are plenty of moments where spending 10 mins on a prompt saves me about 15mins of work. But I have to wait 5 mins for the result, so it ends up being a wash except it has a downside that I didn't really write it myself so the actual details of the solution aren't fully internalized.

There's also plenty of moments where the result at first glance looks like a good / great result but once I start reviewing and fixing things it still ends up being a wash.

I find it actually quite difficult to determine the result quality because at first glance it always looks pretty decent, and then sometimes once you start reviewing it's indeed the case and other times I'm like "well it needs some tweaking" and subsequently spend an hour tweaking.

Now I think the problem is that the response is akin to gambling / conditioning in a sense. Every prompt has a smallish chance to trigger a great result, and since the average result is still about 25% faster (my gut feeling based on what I've 'written' the last few months working with Claude Code) it's just very tempting to pull that slot machine lever even in tasks that I know I will most likely type faster than I can prompt.

I did find a place where (to me, at least) it almost certainly adds value: I find it difficult to think about code during meetings (I really need my attention in the meetings I do) but I can send a few quick prompts for small stuff during meetings and don't really have to context switch. This alone is a decent productivity booster. Refactorings that would've been a 'maybe, one day' can now just be triggered. Best case I spend 10 minutes reviewing and accept it. Worst case I just throw it away.