Ironically, I just completed a lot of testing on this very issue over the weekend for an upcoming league event I'm running from Suzuka. I was always under the impression that weather in GT5 was completely variable on those tracks that allowed it. And indeed, I know in the past, in online lobbies, it's started and stopped raining at the Nurburgring at various times throughout the course of a race. But at the present time, at Suzuka and apparently at Monza, it's not so.
Since last Friday, I spent many, many hours testing weather conditions in my online lobby. And after opening a lobby and resetting the conditions approximately 12 times, here's what I can tell you. (At least at Suzuka)
Roughly 3 out of 4 times, if you select weather as 'rain' but do NOT check 'fixed', you'll enter a wet course and the rain will continue to fall for 3~5 minutes. At that point, it will slowly dry out and within a few minutes the sun will come out. Within 20 minutes of opening the lobby, the track is usually dry enough to change to dry weather tires. Sometimes a few minutes longer, sometimes a few minutes less. Intermediates were pretty much useless because while it's raining, or immediately after it's been raining for a long time and the track is saturated with water, intermediates aren't really sufficient. Or rather I should say, that they seemed no quicker than full wets, even as the track was drying. So you only have a window of 2~4 laps, maximum where they're of any benefit. (This assumes you have the level of grip set for 'real' as opposed to 'low').
Roughly 1 out of 4 times, if you select weather as 'rain' but do NOT check 'fixed', the rain continues to fall, non-stop with no let up for 1 ~ 1.5 hours. (I didn't bother to test it any longer, although I assumed it wouldn't change anymore).
Twice, I also selected Sunny but NOT fixed to sunny and it alternated both times between sun and shade but it never rained. During those two times, I kept the lobby open for about 30 minutes each.
So, essentially, if you're running a long race like we intend (about 2.0~2.2 hours if you include a 15 minute qualifying session). You're either going to be stuck with a wet track for qualifying, which will dry out completely within 20 minutes and you'll end up having a dry race, running in 'sunny' conditions. Or it will rain the entire time with no let up.
It seems like there's no in between.