This is just my theory of when each season of Total Drama took place in-universe. I've tried to follow every canon clue provided, but feel free to correct any inaccuracies you may see.
Let's say that Island started on June 8, 2007, first airing on July 8. After 8 weeks, the season ended on August 3. The campers had a few days of rest and relaxation, with "Total Drama Drama Drama Drama Island" taking place on August 5.
Action started two days afterward, making the date August 7, 2007. This season lasted 6 weeks, ending on September 18. The "Celebrity Manhunt" segment took place on February 18, 2008 (intended to be exactly 4 months after the season's end).
World Tour started immediately afterwards, on February 19. This season wasn't given an official run time by Chris, but let's say it took 7 weeks, concluding on April 8, 2008. Cody's birthday (April 1) was very close to the end of the season, so even if April 8 is not exact, World Tour must have ended in early-to-mid April.
Revenge of the Island began on June 8, 2008, exactly a year after Island's debut. Due to the new 13-episode format, this season only lasted 5 weeks, ending on July 13. Sometime in early August, Chris was sentenced to a year's imprisonment for dumping toxic waste, but thanks to money, influence, and (relatively) good behavior, managed to get out after 10 months, allowing him to begin All-Stars on June 8, 2009.
All-Stars was the shortest season yet, lasting only 2 weeks (due to challenges taking place every day, as opposed to every three days). The season ended on June 22, 2009. Pahkitew Island also took place in 2009, after some time (Chris needed time to set things up). It began on August 8, 2009. If challenges are taking place every day, it will end on August 22, 2009. If they're taking place every three days, it will end on September 12, 2009.
If this timeline is accurate, two seasons are taking place every year, with a new cast debuting each year. At the time of Pahkitew Island, the Island cast is 18, the Revenge cast is 17, and the Pahkitew cast is 16. If Chris was born in 1978, and his birthday is November 18, he is 30 years old at the time of TDPI (just like he said he was). In the TD viewing world, each episode airs a month after the actual events take place. This could potentially explain something from Island- When Courtney talks about her unfair elimination at the Playa des Losers, no one seems to know what she's talking about. They weren't lying- they simply hadn't seen that episode yet. So, that's my idea. As I said, feel free to correct any mistakes I may have made, or just say what you think of the idea.