Bukit Lawang is roughly 85km northwest of Medan, and the journey takes three to four hours depending on traffic and road conditions.
From Medan: Shared minibuses (travel) depart from Pinang Baris bus terminal in west Medan throughout the morning, with the last reliable departure around noon. The fare is around 30,000–40,000 IDR. Chartering a private car from Medan costs around 300,000–400,000 IDR and takes the same time — worth considering for groups or for an early-morning start.
From Medan's Kualanamu Airport: The airport is on the opposite side of the city from Pinang Baris. The most straightforward option is to arrange a private transfer in advance through your guesthouse, or take a taxi or Go-Car to the bus terminal and pick up a shared minibus from there.
There is no direct bus from Berastagi (the highland resort town); change in Medan.