I was once told that a pilot with no IMC experience would loose control in about 40 seconds if s/he entered cloud. I don't know how true that really is, certainly instrument flight has never seemed that hard to me.
I think I've more commonly heard 180 seconds, but that's life expectancy, not time-to-loss-of-control.
Personally with only the PPL instrument training I never had any trouble flying on instruments ... with an instrument rated instructor sitting next to me, ie no risk, no pressure, I could always chicken out and say "you have control". But ... I suspect that the same is true of every non-instrument pilot who has lost control in IMC, so I don't think that proves much. It's what you do when it's unexpected and you're on your own that matters.
I'm now on an IMCR course. The other day I completely lost it, pitch all over the place, overcontrolling up and down and sideways ... brought it back under control, at which point the instructor, who sat there calmly watching throughout with his hands in his lap, said "look at the localiser needle, your heading is wrong". I have no idea what would have happened without the instructor sitting there, even though he didn't do anything - I knew he
would take control if necessary, so I was only at risk of looking silly, not at risk of ending up dead.