Hi Lars. I get continuous growth all summer so there is always green growth at the growing tips. You'll get a spring push and it will harden off in 6-8 weeks - at that point it's ready to be worked. The signs are that the leaves change to a dark green color and the shoot color darkens from light green over most of the shoot (growing tips are the exception).
If you leave the spring growth, the shoots will continue to lignify through the growing season and become increasingly difficult to wire for movement - even if you prune it at some point during the year. So work your tree as early as possible - after you've finished, the tree will push again - you can work that growth after it hardens too. That's the rhythm of developing broadleaf trees for bonsai. In my climate I can reliably get three growths a year on tridents in one growing season - sometimes four. This tree has been worked twice and the third growth is extending now.
Also, be sure to not work your tree too close to autumn. The tree won't reliably respond at that time and you're priority is no longer training, but prepping the tree for the next spring.
Scott