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Personal choice...I dont let it get below 20% usually or 6 months. I should add that my car is a weekend warrior with low miles so the 6 months usually comes first.
I get it changed around 40% myself. (Roughly 4k) Gas milage improves after a 40% change, so it's a good indication it's about the right time to change it.
I do it every 4 to 5k but went 7k last year on a trip and it still said 50% left. I think it's a worse case scenario. So I try not to go the limit. My .02
You'll get a lot of varying answers on this question.
Mine is a weekend driver and I use Mobil 1 (as I assume you do). I have had it 7 months and I changed the oil when I took it to the dealer--ours is a good one unlike some I've read of--for some other work. I think I'll do it twice a year unless I put more miles on it in 1 year total than I have so far. I've put less than 5k miles on the car so far.
I change mine at around 40%. I went on a 9,000 mile trip this summer and when I got back I changed it, Oil Life was 30%. The only exception to this is when I take her out to the track for a weekend of fun. I don't get out on the track with less than 90% remaining.