Depends on the environment. In some places you can throw an apple core and it will be gone in a week. In some places it can take years.

It all depends on the local environment and ecosystem

The truck i learned is to eat the apple from the top down, working your way through the core of less of it into a bite and makes it much less offensive, plus you don't have to finish your apple snack by eating the 'trash' last

Here I wrote some of my thoughts on littering. And here I wrote some of my thoughts on poisoning myself.

In all, my thought is that the cumulative effect of everyone chucking their apple cores on a popular hiking trail would be a negative impact for everyone. And even if the circumstances are different - like being on a more remote trail - I think it is important to maintain the habit and encourage this level of fastidiousness with other outdoors enthusiasts. And really, it is seriously no big deal to just put your apple core in your pants pocket, or hold it in your hand, or exercise the slightest amount of forethought and bring something to use as a trash container with you. Or just eat it.