Honestly, Splash Mountain is hard to figure in the summer but yes, usually between 10 - 11pm is pretty good. On Friday nights I have walked on during that time. Usually at night the lines are pretty short, especially if you go on during the fireworks or Fantasmic. Space Mountain can get down to like 30 minutes or less sometimes. Watch out for Indiana Jones though. Maybe it's because of the new movie....the last time I went which was in June, at 11 at night the line was 75 minutes!!!!
During the day, the line should be fairly short though. Most rides should be well below 45 minutes. Some rides like Pirates and the Haunted Mansion will probably we about 5 to 10 minutes. Peter Pan is always long though...especially at night for some reason.
I wont lie, summer is brutal for Disneyland but I'd say go early in the morning or stay very late and the line should be ok for ya! Don't forget, Fastpasses are your friend!