How to show FPS ingame?

As a variant, you can create another timer, so that he called 1 time after 30 seconds launch.

GetWorld()->GetTimerManager().SetTimer(SomeTimerHandle, this, &AGameCharacter::StopShowFrameRate, 30.f, false);

StopShowFrameRate()
{
GetWorld()->GetTimerManager().ClearTimer(FPSTimerHandle);
}