I've been monitoring my battery usage rate pretty closely since I got the gear S3. On average, I get a discharge rate of about 1.5-2% per hour with AOD turned off, and 2.5-3% per hour with AOD on. Assuming this rate is more or less linear throughout the discharge, this would mean 50 hours of usage with AOD off, and 33 with it on. If I take my watch off the charger at 7 AM, I usually end up at 75-80% power when I come home at 5 pm. For those who are having terrible battery life, I'd recommend checking two things, based on my experiences with the S3 and the S2:
1. Is your watch in constant bluetooth range of your phone? If you step away and hover around the boundary of this range (usually 30 feet or so), your watch will constantly disconnect, attempt to connect to LTE, and reconnect. This causes a severe drain on the battery. Last year my Gear S2 would barely make it 6 hours. Turned out the bluetooth radio on my Note 5 was shot, and the disconnect/reconnect would happen if I were even a couple of feet away from my phone. Exchanging my phone fixed the battery life on my watch.
2. Did you turn LTE to always on? This isn't as severe a drain as the above scenario, since maintaining an LTE connection takes less power than establishing one in the first place. But if your watch is connected to the cell network at all times, then the 2-4 day battery life estimate isn't realistic.