Sliced or mixed fruit stays fresh in a closed container in the fridge for about 2-3 days; that's much shorter than whole fruit, since a cut surface exposed to air starts drying out and browning quickly.
Some fruits, like banana and apple, start browning soon after cutting, while grapes or strawberries hold up a bit longer; a mixed fruit salad's shelf life ends up set by whichever fruit in it is most fragile.
Are there ways to slow browning?
A few drops of lemon juice on cut apple or banana slows the surface's contact with air and noticeably delays browning. Adding fruits like that last, right before serving, also keeps the cut surface exposed for less time.
Using a loosely fitting lid instead of tightly wrapped plastic reduces how much the fruit sweats and releases liquid; moisture trapped in a tightly sealed container can make fruit turn mushy faster.
Which fruits shouldn't be mixed?
Some fruits, pineapple among them, contain an enzyme that can quickly break down and water out softer fruit nearby; keeping fruits like that separate until serving and mixing them in last keeps the salad crisp.
Juicy fruits like watermelon and melon release their own liquid once cut, so pouring off the liquid that collects at the bottom of the container from time to time helps the rest of the fruit stay firmer longer.
Should fruit salad be prepped ahead?
For a fruit salad made ahead of a gathering, the fruits most prone to browning are best added right before serving, while the sturdier fruits can be cut a day ahead and kept in a closed container.
Drizzling a thin layer of juice, orange or apple juice work well, over a prepared fruit salad adds flavor and cuts down on air contact, stretching freshness a little.
Mixing gently matters too when tossing a big bowl of fruit salad; rough stirring bruises the fruit and makes it release its juice early, which hurts both appearance and how long it keeps.
Frozen fruit pieces shouldn't be added to a fresh mix; thawing frozen fruit releases a lot of water and can make the fresh fruit in the mix soften early too.
Sweeteners added to fruit salad, like honey or syrup, add liquid too, so adding those right before serving rather than ahead of time keeps the salad from getting watery while it waits in the fridge.
Adding citrus to a fruit salad with the membranes removed improves the presentation and cuts down on the extra liquid those membranes release into the mix.
If leftover fruit salad is meant for the next day, spooning off the excess liquid that's collected on top keeps the remaining fruit from sitting in an overly watery mix.
Cut fruit has a much shorter life than whole fruit, so cutting only what's needed and eating it soon gives the best result.