"Every 5,000 km or 3 months" is a common rule of thumb, but it's not always accurate. The real answer depends on three factors:
1. Kilometres
Follow your owner's manual. Modern synthetic oils can often last 10,000 to 15,000 km, while mineral oils might need changing every 5,000 km.
2. Time
Even if you don't drive much, oil degrades over time due to exposure to air and moisture. If it's been over a year, change it regardless of mileage.
3. Driving Conditions
If you primarily do short city trips, sit in heavy traffic, or drive in dusty conditions, you fall under "Severe Service." You should change your oil much more frequently than the manual's standard interval.