I did some timing on these...
The battle can be split up into three parts...
1) The time from the moment when the general disappears till the time when the battle starts
Regardless of the type of general this is always 5 seconds.
Part of those 5 seconds is probably the 3.3 seconds it takes for the general to walk to the center of the camp.
The remaining (or preceeding) 1.7 seconds may well be the time that has to elapse till the camp is considered busy engaging the attackers.
If another general is walking into the same camp's influence area after those 1.7 seconds, that general will not be intercepted.
2) The time for the battle itself
This is dependent on the number of rounds, but is independent on the number of phases (first strike, normal and last strike units).
- For a fast general (bhg or veteran) this is 10 seconds per round of fighting
- For a normal general this is 20 seconds per round of fighting
3) The time for the countdown if you are victorious
This is the part where the camp starts smoking and the countdown bar is going from green to red.
This part only occurs if you're winning the battle. If you lost, the time for this part is 0 seconds.
The duration of this part is likewise dependent on the type of general.
However, in addition to that, it is also dependent on the type of camp:
- For a fast general killing a trap it is 10 (2+8) seconds
- For a normal general killing a trap it is 20 (4+16) seconds
- For a fast general killing a camp it is 20 (2+18) seconds
- For a normal general killing a camp it is 40 (4+36) seconds
- For a fast general killing a watch tower it is 80 (2+78) seconds
- For a normal general killing a watch tower it is 160 (4+156) seconds
- For a fast general killing a fortified watch tower it is 120 (2+118) seconds
- For a normal general killing a fortified watch tower it is 240 (4+236) seconds
The countdown will visually not start until 4 seconds (2 seconds for a fast general) have elapsed.
That is the time indicated in brackets above.