2003 Chevy Trailblazer Best time to change brake pads

Generally they MUST be changed at 1/16 or sometimes 1/32.

But I would change them now, remove them and check the condition or the rotor, they should be turned or at least cleaned with brake clean. If there are any groves or uneven wear, have them turned or replaced.
Check condition of calipers, hose connection, slide pins, bolts. Replace any worn parts.

Rear pads generally do not wear as fast as fronts, but if they are 1/4" or lower, replace them.

Use good pads, not the 1 year specials. Wagner and Bendix have a "wear" warranty, so if they wear out you get replacements free. Most other "lifetime" pads are warranted against manufacturing defects not wear. But the wagner and bendix are usually twice the initial cost. So use them if you intend to keep the car for several years.