usually around hours 13-15 is when the rain lets up for me. But it's completely random mostly.
I once raced with 100% rain at the beginning, completely dry by hour 7. Light rain around hours 10-12 and completely dry again but slight overcast for the rest of the way.
The race is at 4pm, sunrise is around 6 am, so you got another 3-4 hours of dark left approximately. Trust me, you're gonna love the sun rise.
When I did the race, I didn't start it until I cycled it to start out sunny. It got cloudier throughout the whole race, and started raining at hour 21 or so. It differs for everybody, I'm sure.