In 2025, that third beta landed on July 7, in line with previous years (it was July 8 in 2024, July 5 in 2023 and July 6 in 2022, for instance). You have to go back to 2021 for a later release, and that was in a year when Covid was still causing issues worldwide.
Each time, the public beta launched between five and seven days later. Last year, the fourth dev beta landed on July 22, with the public beta just two days later on July 24.
I believe this year’s public beta will follow the third developer beta. That beta will likely land on Tuesday, July 7 or Wednesday, July 8. This, in turn, means the public beta will probably go live on or around Tuesday, July 14, ready to be downloaded to your iPhone.