Just starting to check at the moment using 787-10 B787H (Heavy) ORBIT AIRLINES, I will use the template after testing has ended
So, BATT ON
APU STATUS before switching APU to ON
APU switched to ON
Still same status
After switching APU to START position
APU Status stabilized and still ON after 1 minute
APU GEN ON
And Yes, it stops by itself after 2-3 minutes !!! So, we have a limited time to start both engines. 
Here with temperature decreasing :
