Where should i spend apples?

Apples are great for grinding. So yeah, I say keep farming petals and medals. Any leftover apples are best saved for future events.

Training will always be there, but the event will end.

Farm the lottery because this is as good as it gets. There are 600 petals per box. That is 4 million QP (once you have all materials, QP will be the blocker for highest level 5* skills, I need about 1.5 billion QP). You also get 25 4* exp cards and another 15 3*, which is basically 15 MP. I get almost 200 petals every 40AP on luckier runs. 3 runs per apple is efficient.

You should have zero apples.

I'll suggest to keep your apples.
You may ask why. Reason being, there is a Christmas lottery, two in fact this year. One, is a rerun. That one is limited. The second, actual run has unlimited boxes.
"What's so special?"

Two words: Skill Gems

The lottery boxes give you skill gems, which are incredibly useful materials for skilling up your servants. So save up for those lottos, because you'll want to get as many as you can.

That being said, this lotto is worth spending apples on, but not as much as the Christmas one.