How often do you feel like you’re out of ideas and“guessing” in the lab?

A friend in a chemistry PhD program described research as sometimes reaching a point where, after you’ve tried the obvious fixes, it starts to feel like you’re just guessing. I’m curious whether this is a common experience among researchers, though.

I.e., encountering situations where you’ve already checked the protocol, reviewed the relevant theory, applied heuristics, tried all the promising approaches you can think of, and talked through the problem with others — but nothing seems to fix the issue, forcing you to make speculative guesses.

How often does this happen in your experience? For example, what percent of projects does this occur in? When it does happen, how long might you spend in that “guessing” phase before finding a solution (i.e. is a couple months not uncommon)?