I used to leave about 10 a.m. a little behind traffic so I could get into town early enough to roam around a bit and get settled in the hotel. It s about a four to six hour drive give or take depending on traffic and where you are coming from in LA.
I will leave as early as possible. You will be going against traffic heading out of LA. 9am - 10am. LA -Vegas traffic starts building up around 1pmhttp://www.lasvegas-how-to.com
I would say if you leave about 11am you should get there about 3pm, if your stopping at Baker to get gas and eat which i usually do, then expect around 4pm..
I moved from southern CA to Las Vegas. If me I would leave right after the a.m. rush. About 10am.
As soon as possible as you ll miss all the fun.