Gunung Batur rises from the bottom of a volcanic crater which also contains a lake. The initial view one gets from the crater rim is quite superb - almost the whole circumference of the crater is visible, with the lake and mountain within. On the west side of the mountain can be seen recent black lava flows. Batur has three main crater vents itself; the peak is around 2000 feet above the lake.
The best time to walk up Batur is just before dawn, which is around 05:30 to 06:00. Unlike many peaks, the view from Batur at dawn is well worth the walk. Bali can be seen in all directions; Agung seems no distance away and Rinjani on Lombok is very distinct. Not long after dawn the haze and the brightness of the sun obscures the more distant objects from clear view. More importantly, by 7am it is likely to be distinctly hot (from the perspective of British people), and it would be uncomfortable to be still ascending later than this.
The worst aspect of Batur is the gaggle of inisistent "guides" who work out of a hut at the start of the path near Pura Jati (temple) and who claim that it is essential to have a guide for safety reasons and in case you get lost. They want an exorbitant amount per-person for their assistance, which in reality is hardly required. The ascent is only 2000 feet, and even by moonlight the peak is clearly visible almost every step of the way. The path is well-worn and not difficult to follow - take a torch if its still dark.
If you get a lift down to the crater floor, most drivers will take you to the guide hut, where you will be ensnared. To avoid the guides, take the alternative road which skirts around the guides' hut - the path up Batur crosses the road in a gentle dip, and is hard to miss.
My plan was to start walking some time after the guided tours, expecting to catch them up at the summit, where the guides could not accost me. This worked fine. In fact I walked on the road from Penelokan to the path (1hr), then followed the path to the summit (1more hr), returning to the road via the 2nd and 3rd craters (1 more hr), where I hitched back to Penelokan in a lorry.
I was in a bit of a hurry, as had a bus to catch later that morning, but I would say that reasonably healthy people could walk up the path in 1.5hrs, at least as far as the crater viewpoint , if not the final steep part to the peak.