Roughly how long should I cook 3.5kg pork belly ribs in electric smoker?
Based on your comment, you are basically barbecuing pork belly with the ribs still attached.  I like the approach these folks take.  You will want to maintain your smoker temperature at 225F (107C).  You will also want to use a probe thermometer.  They suggest treating the belly almost like one would treat brisket; that is, when the internal temperature reaches about 165F (74C), to wrap it in foil.  Then, continue until the internal temperature reaches 200F (93C).  Their belly took 8 hours using this method.  Given that yours still has the ribs attached it will likely take a bit longer.  It's hard to estimate time beyond this.  You will want to monitor temperature.  If it is done early, just keep it wrapped and place it in a cooler.