Change brakes or not?

hi, I bought some brakes for my 92 toyota paseo, but today just to make sure I should put some new ones on, I went to my local mechanic that has worked on my car for about 6 years, and he looked through my rims and said that it looked like I still had 60% life in my brakes, but about 2 minutes ago hs istant said 40%, but the guy who said 60% is the main mechanic(the smarter one). How can they tell I have that much left, I barely see a small gab in between the rotor and the caliper. Should I change them, or not, I only drive around 6,000 miles per year, and my brakes are not making any odd sound and work great.