When should I unplug my macbook air off?

I just got a macbook air and I'm trying to preserve the battery, for that, when should I unplug my macbook off it's charger? Most sites say not to 100% but they don't say to which percentage is ok, I've heard some say 80% but I think that's not so much, I'd rather something like 95% so I feel like I'm charging it a lot, but not filling it up unnecessarily, although, it's not up to my liking, but how it's best for my macbook, maybe 90% is ok? I always plug it in when it's at 25%.