Since there's been no official announcement yet, all we can do is make some educated guesses.
The first three Charge devices came out about every two years: November 2014, September 2016, and then October 2018. Following that cycle, the 4th iteration should have been released around September or so in 2020, but it instead arrived in March.
If we assume the Charge 5 is coming 1.5 years later, we’re looking at a Fall 2021 timeframe. It could arrive as late as November or December. There’s really no telling given this odd release pattern, but if only a few changes are expected between the Charge 4 and 5, we might see this new Fitbit even sooner. That said, Google completed its Fitbit acquisition in January 2021, so a release date is basically up in the air at this point.