I heard they were hoping to have a single lane in each direction of I-15 open by the weekend. That might be optimistic though. I also heard that the passenger vehicle route through Valley of Fire is much quicker than the other route that commercial vehicles must travel (where you detour from Cedar City and over to 93).
Google maps says it adds an hour to the drive without traffic. However, I can't really imagine a scenario where there won't be horrendous traffic even if they have one lane open on I-15. Best bet might be to stay over night in St. George and then leave extra early in the morning to avoid as much of it as possible. On a good day with no traffic, it's about 5.5 hours from St. George to Anaheim. So I'd plan on it taking your 7-8.5 hours.
The other bonus to leaving as early as possible is that you hopefully will get to Southern CA before rush hour there.
The hotel we generally stay in St. George is the Hilton Garden on the south end of town. But there are lots of other options.
Good luck! My DH is actually working in LA this week and has to make the drive back sometime next week so I'm hoping it's taken care of soon too.