Imtiaz sb... it depends on the usage of car... My car (COROLLA 2007) coolant is ALHAMDULILLAH still in same condition from the day I got hand on the car (at that time car clocked 88000 KM), no color change, same as some what sticky to hand and after that uptil now in almost 5 years I drive it almost 65,000 km (currently odo reading is 145,000 KM) ...
As far as my other car (CRESSIDA 1982), I changed its coolant almost every two years or less or when I see its color is changing and very less sticky, because I dont drive that much, hardly it completed 5000 km in 3 years after death of my father (may his soul rest in peace)
If its not some what sticky, color is changed its better to change the coolant.. and try to go for good company coolant or Toyota Genuine coolant...
For further and more clear details and suggestions we must have to wait for Xulfiqarbhai and VCheng and other professionals.....