It shouldn't be timed in any precise/mechanical way. This entire passage is very tender/lyrical, so think about how the turn contributes to the overall shape. Let it sing. But, be careful not to have what I think Tovey described as annoying mannerisms in the tempo... there are only two ritards marked in the entire movement, so whatever you're doing, don't exaggerate it or draw too much attention to it. This movement is deceptively difficult, almost in the extreme, to play musically and to play well. I've been working on it, plus the other three, for 8 months now, and I still have problems getting a good performance out of the first movement that I'm satisfied with. In a way the fugue is easier. Keep at it.