This means:
“How long can a watermelon safely be kept after it has been cut open?”
Breakdown
“Once you have cut it” = refers to a watermelon that has been sliced, cubed, or otherwise opened.
“How long does it last?” = asks how long it remains fresh and safe to eat.
The question is mainly about food storage and spoilage.
General guidance
Once cut, watermelon should be refrigerated promptly in a covered container or wrapped well. A commonly recommended guideline is to use refrigerated cut watermelon within about 3–4 days for best safety and quality.
Don’t leave cut watermelon at room temperature for more than 2 hours (or 1 hour when temperatures are above 90°F/32°C).
A simpler version:
“How long can you safely keep cut watermelon in the refrigerator?”
