A lot of times you can get by with Bluetooth. Of course, one then needs an app and a phone or other host device but the power consumption on Nordic Bluetooth chips is just miniscule.
it saves persistent variables to non-volatile storage and deep-sleeps at 15-20uA according to my power profiler 2, waking up on a timer
when it wakes up, connects, checks battery, and updates display, it can peak 350mA, but mostly stays around 100-150mA while it works
I haven't timed it super precisely (my off-state is 30 min and my on-state is 4-5 seconds), but definitely going to check this for thoroughly in my next project, which will wake from deep sleep on button press; curious if there's some delay as everything powers on.
As far as bluetooth on Watchy goes, it's more useful for OTA updates than anything else. When in this mode, Watchy's firmware can be sent as a .bin over bluetooth. This takes only a few minutes and then the ble radio is turned off. No real hit the battery.
The other [quicker] way to flash the firmware is via usb, during which time the battery is charging.
What I like about Watchy is that it requires no phone or apps, is running firmware I wrote and flashed, and occasionally calls my server for data if I need it.