There are two main ways to tackle Gunung Rinjani. The one described here is a round trip, starting and finishing in the small village of Senaru, to the north of the volcano. This is handy for those without arranged transport, but it does mean doubling back on yourself and adding an extra day, so if you're pushed for time you might want to consider the second option. This follows the route below for the first two days, but then on day 3 you climb to the summit and then descend southwest of the volcano to the village of Sembalun, where you should be able to catch public transport back to Senaru. You can also do this option the other way round, from Sembalun to Senaru; there aren't any rules here, as most of the time you'll be following your instincts rather than the map.
Talking of maps, I didn't really have one. I hurriedly copied a map down from a piece of paper I found in Senaru; as you can see from the picture above it was pretty basic, but nobody seemed to have anything better. I think some of the latest guidebooks to Indonesia have maps of the walk as well as all sorts of handy information (like distances and times), but although this makes life a lot easier, it's possible to do the walk without much information. After all, once you reach the rim, you can see where you need to go.
If you have the time I recommend the more relaxed four-day schedule below, as this gives you time to spend a night in the crater itself, camping on the shores of the idyllic crater lake. You cross the crater on day 2 even if you're heading through to Sembalun, but spending the night in the crater is worth the effort, despite the bugs. Here's the route I took:
|1||Senaru to Crater Rim|
|2||Crater Rim to Summit Base|
|3||Summit Base to Crater Lake (via Summit)|
|4||Crater Lake to Senaru|
There's no reason why you couldn't spend more time in the crater area. There's a spring from which you can collect drinkable water not far from the hot springs (the locals will direct you there) so the only concern would be food. Just make sure you have the strength and provisions to get out again; you wouldn't want to be stranded in this part of the world without anything to eat.