Q1.
The DECU has inbuilt timers that count cumulative time limits to some algorithm programmed by an engineer that a pilot need not know about.
Q2.
The real answer to your question is you use as much OEI power as you need for as long as you need it. If you need 30 second power for more than 30 seconds, use it. The DECU does not stop you using 30 second power for more than 30 seconds. Same for the 2.5 minute limit.