stargazer48

I posted this in the Bicycle Mechanics forum but thought I would also post it here.
I returned to biking at the beginning of the year and bought a Zizzo 8 speed 20" folding bike. After 3 months, I was primarily using the 8th gear at least 60% - 70% of the time. I even changed out the 48T chain ring for a 52T because I was spinning too often on flats. After 1700 miles, I mistakenly though I needed a new chain because the chain started to slowly drop off. When I installed a new one, it skipped on the 8th gear only when I pushed the pedal hard. I re-installed the old chain and the skipping of course stopped. I got a Park tool chain checker and tested several sections of the old chain and the wear was not at 75% yet. I didn't lube my chain until about 800 miles. My question is, since I use the 8th gear (11 cog) quite a lot, how many miles can one get on that gear before having to replace it? The Zizzo people were kind enough to send me an 11T cog gear so I am ready to install it when the chain is at 75%.