If it were me, I would easily allow 1:15 (give or take 15 minutes). An hour may be doable, but I would not expect to do it in less. (Also depends on whereabouts in Santa Monica you are heading. Surface street traffic can crawl slower than freeway traffic, so getting from the 10 to your destination could take an inordinate amount of time.)
Even without accidents and closures, the route taken and hour of the day is crucial.
Route: Going by the most direct route (5 North to the 10 West), you are still talking about 40 miles of freeway driving. You are unlikely to maintain a speed of 60mph over the entire route, regardless of time of day. There are other routes that, though longer, could take less time overall -- but it's really a crapshoot.
Time: Anytime on a weekend will probably be faster (barring any routine Caltrans work, which is scheduled more on the weekend). The earlier the better. On nice days, you can expect more traffic on the weekends, as people head to the beach. Still shouldn't be as bad as a weekday, however.
Weekdays will be rough, regardless of time of day. Even if you avoid rush hours, these are still two very heavily-traveled freeways. If you have to go on a weekday, your best bets are probably mid-morning (after 10AM), early afternoon (1:30 to 2-ish), or later in the evening (after 8:00PM).