When I was relaxed, I'd probably been using Pantene for about 8 years. I originally used their "dry hair" formula. Then I switched to their "smooth and sleek" line and then tried out the "daily moisturie renewel" line and liked it better than smooth and sleek. I used the dry hair formula and smooth and sleek for the longest.
I also use Aveda Anti-Humectant for about 5 years once I could actually afford it. I only had to purchase it twice a year so it was no big deal.
The brand that I can remember using the longest is Mizani but I can't say that I was faithful because I always tried new brands in between. So far I've been faithful to Kerastase for the whole month of July.
Blue Magic Grease - 94'
CON Professional Poo - 94'
Fantasia IC Reconstructor - 95'
Olive Oil - 98'
Affirm Relaxer - 99' (with the exception of one affair)
CON shampoo for at least 15 years
CON relaxer for at least 12 years
Nexxus: humectress, headress for 15+ years
Sebastian: red-ality, penetrait, 2+1 for 15+ years
Hellman's Mayo: since forever.