No you cannot freeze leftover salad because the water content in the lettuce is so high and freezing the leftover salad will cause the salad to become soggy and not good to eat.
You could freeze the salad but it's not a good thing to do as the salad won't be as good when you thaw it out.
It's best to just refrigerate the leftover salad and eat the salad within a few days or at least within a week of refrigerating it.
Leftover Salad will last for up to 7 days in the fridge before it goes bad.
Of course if you have meat such as chicken in the salad then it may spoil sooner but you can usually leave salad in the fridge for up to 7 days.
Although it's better to eat the leftover salad before 7 days so it's at it's freshest.
A salad will last in the fridge for 3 to 7 days.
If you have any meats such as bacon, ham, chicken etc then the salad will spoil faster than it would without the meat.
Salads that are made with meat should be eaten within 3 days of refrigerating them as the meat can spoil and make you sick.
If the salad has not meat and is just a vegetarian salad then the salad can last up to a week in some cases.
However it's best to eat the salad within a few days of refrigerating it.
You can freeze salads to keep them longer but if not frozen properly the salads can become soggy.
I always just makes fresh salads and only keep the salad for a few days at most.