I would think anything more than a couple inches is OK. I just redid my bathroom, and I had a PVC drain pipe a little too close for comfort to the top of my halogen recessed fixture in the shower, especially with the shower trim trapping all that heat.
I just cut it into strips and wrapped it around the pipe. I'm not sure how the adhesive would react with the copper and black pipe though.
Features & Benefits
Aluminized Heat Barrier Is Made Of Woven Silica w/ Flexible Aluminized Finish, Highly-Reflective Surface Of Material Is Capable Of Withstanding Temperatures In Excess Of 2,000 Deg F; Use On Stardard Rivets w/ Backup Washer Or Weater Strip Adhesive; 12" x 12"