Specific to Grapes, published by UC Davis Postharvest Research and Extension Center:
Brown, dried-out stems even when the berries look fine“Berry firmness is also an important factor for consumer acceptance as are lack of defects such as decay, cracked berries, stem browning, shriveling, sunburned or dried berries, and insect damage” — UC Davis Postharvest Research and Extension Center
Brown berries with loose skin, then fuzzy grey mould“Gray mold first turns berries brown, then loosens the skin of the berry, its white, thread-like hyphal filaments erupt through the berry surface, and finally masses of gray colored spores develop.” — UC Davis Postharvest Research and Extension Center
Berries dropping off the stem when you lift the bunch“Shatter. (Loss of berries from the cap stem) In general, shatter increases in severity with increasing maturity, i.e., the longer the fruit remains on the vine.” — UC Davis Postharvest Research and Extension Center
And the general signs for fruit:
Mould or fuzzy spots
Mushy, collapsed, or leaking areas
A fermented or alcohol-like smell
Slimy skin
The first list is quoted from UC Davis Postharvest Research and Extension Center, with the page each line came from linked beside it; the rest are the general signs for fruit. Never taste something to test it — when in doubt, throw it out.
Air and warmth are what shorten an opened item's life — the right container buys back the days the packaging gives up once the seal's broken.
About 1 week at 40°F (4°C) or below, according to USDA FoodKeeper v128. That window runs from the day you bought it — FoodKeeper publishes no separate after-opening figure for this one.
Do Grapes need to be refrigerated?
Grapes are shelf-stable at room temperature: cool, dry pantry, or refrigerate at 40°F (4°C) or below to keep it longer. Refrigerating it mainly preserves quality rather than safety.
How can you tell if Grapes are bad?
Brown, dried-out stems even when the berries look fine; Brown berries with loose skin, then fuzzy grey mould; Berries dropping off the stem when you lift the bunch. That guidance is published for Grapes by UC Davis Postharvest Research and Extension Center. Beyond that, the general signs for fruit are: Mould or fuzzy spots; Mushy, collapsed, or leaking areas; A fermented or alcohol-like smell; Slimy skin. Never taste something to test it — when in doubt, throw it out.
Can you freeze Grapes?
Yes — USDA FoodKeeper v128 gives about 1 month in the freezer at 0°F (−18°C). Frozen food stays safe indefinitely; that window is about quality, not safety.