It's now, as the first pre-release of #kernel 6.18 is out – which leaves plenty of time to find, report, debug, and fix any problems that Fedora might encounter when it will switch to 6.18.y in about eleven weeks. And testing is not even hard: just use the mainline-wo-mergew copr (https://copr.fedorainfracloud.org/coprs/g/kernel-vanilla/mainline-wo-mergew/).
In case you want to play it a bit safer, then testing in two or three weeks is still great! But you really should be testing 6.18-rc6 (five weeks from now), as Linus by then wants all regressions fixed. But at the same time there is still enough time to report, debug, and fix many problems before they reach Fedora – or Arch Linux and openSUSE Tumbleweed, that just like Fedora regularly switch to the latest upstream series.
Testing a newer kernel series only one or two weeks before it enters Fedora[1] is way too late: most bugs can't be fixed anymore before Fedora has to switch to the new series, as the previous series by then is usually obsolete and unsupported upstream.
[1] e.g., either as part of a "kernel testing week" or through the "updates-testing" repos – this is how the update to 6.17.y will soon be prepared for #Fedora 41 and 42, which right now are still on 6.16.y, which was EOLed yesterday.
