This depends on what type of setup you’ve got.
The water in non-filtrated bowls generally need to be changed every week, depending on the size of the bowl. If your bowl is more than 3 liters and you don’t overfeed your fish you can probably leave it a bit longer.
Never let the water get murky. If it does after just a few days you’ve got problems. Common causes include:
- Over-feeding – feed no more than what your fish can consume within 5 minutes
- Sunny location – always keep your fish away from any direct or indirect sunlight