Last night I was checking the battery usage and for me the average drain of battery for the "Home & Lock Screen" is 14%. I have an iPhone 5, which has around 9-10h of battery life. This represents 1-1,5h of battery for this use case.
People are worried about the battery life of the watch, that it would be awufull if you went without battery during your day. That is not your "problem".
Back in the day, Aple said that the mac was your "digital hub". It would sync all your devices through the "swiss knive" called iTunes. Nowadays the "digital hub" has changed to the iPhone. It's our main device and for some, the only one. This is the device that needs to last you all day and here is where the watch may help in that task.
180 interactions seems to be the average interactions a user has in a day of usage of their phone. Now with the watch, you can preserve the battery life of your phone in many of those interactions.
The watch, at last this model, will be a premium notification center. If its battery life goes to around 180minutes, it gives will you 120 interactions of 1 minute - the other third I will give it to background activity.
The only problem is that this extra battery life for your phone will cost you a lot of money.