Hi Nicolita,
We generally recommend brake pad (or shoes in the case of drum brakes) replacement at 20-25%. We make that decision on whether the brake material will be sufficient to reach the next service interval. On cars we service at the suggested service intervals of 7,500 or 10,000 miles, we can estimate the remaining mileage quite well.
As far as replacing the brake discs, there is a 'wear limit' for brake disc thickness particular to the particular vehicle. If your discs are worn to that point, they must be replaced. IF they are not worn to that limit, we would resurface the discs, IF they will still be above the service limit after resurfacing.
I would guess that you require a standard front brake job of new pads, resurfacing the discs, and cleaning and lubing the sliding mechanisms / hardware.
You will not experience a degradation in braking peformance as the pads reach 0%, HOWEVER, if the brake friction material is worn away and the steel backing plate contacts the discs, you will escalate your expenses to include new discs.
Hope that answers your questions.
Feel free to call us for more specific advice and / or quotes.
Steve