How Do I Know if my Dentist Filed Down my Teeth Excessively for Porcelin Venneers?

This is a difficult question to answer that I get a lot.
It's based on a lot of things like:
- Condition of your previous teeth (i.e. old bonding OR decay)
- Amount of crowding that was there
- Your objectives for new smile.
It is unfortunate to report sometimes unncessary tooth structure can be removed if the clinician does not do veneer cases a lot. Meaning they'll just remove a lot and allow the dental ceramic laboratory to "make it look good".
Again, this one is only impossible to tell and you'd need to have before pictures AND x-rays pre-operatively and then post-operatively. Then have that reviewed by an experienced cosmetic dentist.

It is difficult for a patient to determine if their teeth have been reduced excessively. Two visits of four hours each does seem excessive. That is not to say that fast veneer procedures are better.
Preparing veneers properly takes time, the amount of time is dependant on how many teeth are involved in the therapy, the alignment of the teeth and the health of the teeth. If teeth are mal-aligned and or have decay it may take longer. It is best to prepare the teeth in one visit regardless of the complexity of the case. This reduces the possibility for the need of root canal therapy due to the fact that the teeth are exposed less.
My suggestion is, if you suspect that something is wrong, seek a second opinion. If the pain persists, you need to notify your doctor. Tooth sensitivity after the procedure is common, but should not persist past a couple of weeks, and in extreme cases, past one month.