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I don’t use timeouts but the rule of thumb is time out lasts as long as their age. I.e. a one year old is in timeout no longer than 1 minutes. A 5 year old is in time out for no longer than 5 minutes. Etc
My kid is 15, almost 16 months and I have started to recognize when she is doing things she shouldn't be deliberately, just to see what my response will be. I think that's about when you'd want to instate some kind of "consequence", because if you're just saying "no" without anything behind it, that's not beneficial and they'll just keep doing it. What I've started doing is sitting her down for a minute or two, getting down with her and explaining to her why we don't do that, and that she has to listen when we tell her no. I don't really think of it as a "punishment" but it does disrupt her flow enough to get her to pay attention. I only make it 1-2 minutes max, just long enough to explain the thing to her, then we move on and distract with something else. I can't say this would work for every child, but for us, I have noticed a huge reduction in her deliberately doing things she shouldn't be already. She doesn't even touch the fireplace anymore and it's only been a few weeks since I started doing it.
When they understand what they are being put in time out for. So maybe 4.
ASAP
I don’t do timeouts they are not beneficial