Well, I started at 1 minute and I 'moved up' to 2 minutes as I went along and my powers of concentration increased.
By now (I've been at this 'learning morse' thing for a little while) I've got a fairly good handle on judging my own ability. I tend to stay on the same lesson until I'm getting 90% (or pretty close) on a fairly regular basis.
Over the couse of the lessons I've noticd that if I moved on 'too quickly' I've then taken longer on/more attempts at the next lesson. So I think it all evens out in the end.
The thing is that if you can do well just relying on short term memory
you can wizz throught the first few lessons.
But then your short term memory, which is a bit more plastic than long term memory
starts to get mixed up a bit and triggers at the wrong time
which causes even more disruption as you relearn things all wrong.
About lesson 10 ( YMMV etc, but that seems to be commonly reported lesson number ) chaos ensues in the STM
you "hit a wall" because you just unlearned a lot of stuff you thought had sunk in properly . . .
This means you have to start again - this time making sure it does sink in correctly,
if you don't get fed up and give up that is . . .
If possible try to build an automatic "unconscious" reaction:-
hear code -> chars appear in your mind as if from no-where.
You can't easily tell what you have done until ye wall appears and stops the job . . . or not
Too slow = waste a bit of time.
Too fast can = give up.
Did I say YMMV etc?