Thank you for an excellent question.
Thank you to all of you, dear travelers, for responses and sharing your experiences. :)
During the winter, anytime is good to do a tour on Sunday, but it might be a little cold before 10 am to drive in a convertible; after 10-11 am, the weather gets warmer, and a beautiful Californian sun comes out.
Summertime might get busy in Santa Monica and Venice since everyone starts coming to the beach on Sunday, usually around the afternoon.
1-3 pm, it can get hot. However, we provide hats and sunscreen.
Taking this all into consideration, we always do our best to avoid or minimize the traffic and show you the best of Los Angeles during the tour. We recommend a 4-8hrs tour.