The best time to see lava is at night. However, you may or may not see flowing lava. Anyone who tries to tell you they can guarantee it or predict it is lying or trying to sell you something. Your best bet is to call the visitor center at the national park before you head out; even at that you could be pleasantly surprised or utterly disappointed. Keep in mind that the volcano is a pretty amazing sight even if it's not spewing fire.
The hike out to the closest viewing station to the flowing lava requires roughly three to four hours each way—the best way to handle it is to head out in the late afternoon, arrive at the viewing station by nightfall, and then start the trek back when you've had your fill of Pele's fireworks. Bring a flashlight and be prepared for some rough going over the lava fields at night.
There are viewing stations along the way as well, so you don't necessarily have to make an eight-hour roundtrip journey. In fact, depending on how the lava is flowing, a viewing station farther from the lava could actually afford better views.