You need to involve your attorney very early in the process to properly protect your interests. To answer your question though, I'd say as much as the owner is willing to agree to. You don't really say how much your lease terms are, what's included in the lease etc. If you've been paying all the expenses (utilitites, water, trash, condo fee, parking etc.) then you could expect a greater amount to be credited. If the owner has been carrying the fixed costs of the unit (insurance, water, taxes, condo fees, parking etc.) then there might be less room to negotiate.
Your depreciation argument might get countered by the owner saying your directly financially responsible for the depreciation (wear & tear if excessive (this is subjective) etc.). You might want to research how much they paid, how much they financed and try to get a grasp on what the owners financial position is, coupled with the current market value & your ability to get funding.
Lots to cover, more info really needed to flesh out your questions
An agreed purchase price is agreed to at the onset, not after the buyer's wear and tear on the property. Therefore, the buyer does NOT benefit from financially from his own wear and tear. You may want to consider some protection for yourself, that if in fact they eventually back out of this agreement that excessive wear and tear would be financially addressed, once again, in writing through an attorney.
I hope this works well for you, Let us know