Fortnite Season 3 is only a few days in, with the planned end date being on August 27th, 70 days after the launch, but it seems Epic are already planning on delaying the season until at least September. So far in Chapter 2, which began in October 2019, we have only seen 2 completed seasons. By the start of Season 3, if the seasons ended at the correct times, we would have been in week 4 of Season 4.
FireMonkey recently found code in the in-game files for challenges planned in weeks 11 & 12; a season is only meant to be 10 weeks long. This could reveal that Epic is already planning on Fortnite season 3 being a minimum of 12 weeks long, taking it to at least September 10th.
There isn’t really any reason why these challenges would be there unless an extension is already planned. The last time a season ended on time was Season 8; we are currently on Season 13, and the 2 seasons in Chapter 2 are the first ones to reach over 100 days in length.
At least this season is a lot of fun, and hopefully with regular content, it won’t drag on too much if they do extend it. But it would make sense to just make the season the planned length to begin with rather than telling players it is extended later, which usually causes backlash.