You need to train when things look grim so that you're ready to hit the upswing when it happens.
No I think that is bad advice.
I think the best advice is to time it that you START training when you can actually see things recovering; ie fATPL getting jobs. Its not like in one day we go from no jobs to flood gates open. It takes around 2yrs from the first trickle of fATPLs into airlines to full on recruitment. In those 2 yrs you could quite happily get the training done.
Starting when things look grim is STUPID. Whats to say they still look grim for when you finish, or for the many months/yrs after you finish...