Sigiriya by car is the best way to get there. On the road, you’ve got two routes that take you to Sigiriya. The traditional route takes the A01 (Kandy Road) up to Ambepussa where you turn off to Kurunegala on the A06. If you’re lucky and there’s no traffic, the drive should take about 3 hours tops.
It will take you 3-4 hours to get to Sigiriya on a normal day. It all really depends on your luck with the traffic.