Approximately 1-2 hours for a casual stroll and viewing the main buildings; longer if including a meal or market visit.
A superficial visit to the Royal Sittard-Geleen Square, involving a walk around its perimeter and viewing the facades of the main buildings, typically takes about 30 minutes. To fully appreciate the architecture, read informational plaques, and perhaps visit the interior of St. Peter's Church (if open), allocate 1 to 2 hours.
If your visit coincides with a market day (Thursdays and Saturdays), or if you plan to enjoy a meal or drink at one of the surrounding establishments, the duration will extend accordingly. For a more comprehensive experience that includes exploring nearby streets and shops, a half-day might be appropriate.