Finally I have sleep modes working on Time Machine Mk. 8. I don't have a current measurement device, so I don't have an exact way of measuring power consumption so I could only conduct very simple, short tests. But here are the results:

1) Display off, press a button to display the time for 2 seconds. Watch is in deep sleep mode when display is off. Running at full 48MHz when displaying time or any other function

-> estimated battery life of around 50 hours, give or take 10. About 12 readings per hour (once every five minutes). Adjusted time once and didn't use any other function.

-> about 600uA current consumption I think?

2) Always on display. Watch is in deep sleep for 990 milliseconds, only wakes up to refresh the display with new info and immediately goes back to sleep. display refresh and other features use full 48MHz.

-> estimated battery life of around 17.5 hours, give or take 5. Similar use to above, didn't use a lot of the watch's features.

-> around 2.5 mA of current consumption?

obviously, the current consumption even with deep sleep enabled is very high. My target is about a week of battery life, which I think is definitely possible. However, I'm still really happy that TM8 can now last almost a full day of use even with an always-on display!