Timer start when you gain control on Decepticons or Autobots, complete any mission on the first level and go for the boss battle wich means

Autobots: The Suburbs in any chapter to fight with Megatron (The Ultimate Doom chapter)

Decepticons: Blackout in any chapter to fight with Optimus Prime (Day of the Machines)

Full Campaign: do both and start with Autobots and finish Decepticons as long as you follow the rules

You can do with any savefile

Cheats Codes are banned

https://www.speedrun.com/tftg/run/zgwg3jny

https://www.speedrun.com/tftg/run/meo9333z