I changed mine at 40-50,000mi because I thought I needed a new water pump. Turned out I didn't need the pump as it was another issue, but the cogged right idler near the water pump failed within 1 year on my replacement Gates kit.
The centre part collapsed and the idler was making contact with the engine block. How did I know? The engine was shaking on start-up one day. I shut it down and checked the timing belt right away. The timing was off and I dodged bent valves. The idler was NSK Made in Japan. Gates is a Subaru supplier in the US. Good luck getting any response from Gates,although I live in Canada. Some Gates kits will have Made in China idlers.
?My OEM belt was about 10 years old and still looked good. MRT Performance in Australia recommended changing them based on time as well (every 5-8 years I think) in one of their videos.
To do it again I would go with a Mitsubishi belt and an Aisin kit.