Honestly, it depends on what iPhone you have and the age of that phone. If the phone is an iPhone 5, it has a smaller capacity battery, and batteries degrade over time, so there is even less capacity. My 6s+ will be on nearly 50% with the watch connected when the day ends(6am-6pm). However when I had my 5, it would die so early into the day(6am-11am). It all comes down to your phone and it's age/usage history