How long is the drive (during the summer) from LA to yosemite ?

About 5 hours. Time of day, and which part of LA you are leaving from will also factor in.

what do you mean by "LA"? Where you are starting from could make an hour or more difference in your drive time . . . . .

from beverly hills

From beverly hills, leaving at about 7 am.

To Fresno, where the Highway 41 into the park cuts off, is about 4 hours, maybe a little less. Then, almost 2 hours into the actual valley. You will get to the park border about an hour out of Fresno, then it's quite a bit farther through the forest before you get to the real scenic spots.

Day of week makes as big a difference as time of day.

If it's 7am on a weekday, allow at least an hour additional for getting out of the LA basin.

We were just in Yosemite last month.
From the LAX area, it is 4 hours to Fresno. From there on Hwy. 41, it was 2 hours to the Valley floor. That was driving on a Sunday and returning after PM work traffic hours on a weekday.

6 hours with 1 pit stop (fast food, toilet break). Do it every bloody year. Don't forget to keep your camera out and ready after you hit hwy 41. I have this awesome pic of the barn painted with the American flag right before the gas station, definitely photo-worthy. Then, of course, once you shoot through the tunnel and the awe factor as you come out. Make sure the passenger is the one taking the photos!