The hike, begins at 2am (you may need to get picked up earlier from your hotel, my pickup was 12.30 from Candidasa), and there's no guarantee that you'll be able to make it to the top, depending on weather and fitness. When you get back to the bottom will depend on your fitness levels. There are no real set paths, the route follows a rain gully, a solidified lava flow, and then a bunch of grafitti up a huge lava field. The last part is the most dangerous to the inexperienced, as cloud cover can limit visibility and make it difficult to find the route to the top...
I went in the wet season, and its possible to get hit with thunderstorms during the hike, in which case its probably wise to abort for safety. Its not a great place to be caught without shelter...We spent some time about 3/4 of the way up deciding whether we should continue or not, as we got hit by a series of small squalls. In the end we did proceed and were rewarded as the clouds on the peak cleared away, and we got beautiful views of lower cloud formations rolling along the South Bali coast.
On the way down we met a procession from the temple, they were quite a humbling group, headed to the top to pray. Adults, children, grannies, all dressed in the ceremonial finery, wearing flip flops, and some with huge baskets perched atop their heads. Made what seemed like an epic journey feel quite a small feat.
All in all a great day, some drama, tension, physical exertion, and all capped off with being reminded that no matter how proud you feel, there are others that can do it better... Next time I'd like to do it in the dry season... With some friends :)
Be VERY wary of inexperienced guides, it is very possible to get lost and people have died on the volcano.