Large-scale fantasy epics spanning many books are pretty common these days. The temptation for budding writers and – believe me, I hear from a lot of you – is to write just this sort of thing. What I want to do here is to give a warning: be very careful about venturing on the longer and perilous road to Mount Doom (or wherever). Let me give you three reasons.
The first is that an epic saga of say 600,000 to 750,000 words is not simply a scaled up version of a 100,000 word novel. You don't just keep writing longer! The best parallel is in architecture. The novel is a house and the multi-volume epic a cathedral. To go from the first to the second is not simply a matter of scale. You need a much larger and more complex architecture. The saga needs to have a very different structure; with such things as a different pacing of high points and the relaxing and tightening of the plot. Rather like any traditional symphony, all the main components (the however many volumes there are) need to work separately. The practical implication is this: I don't really think you can just start writing with the intention of ending up with a satisfactory creation in a few years time. At very least sketch it out!
Secondly, staying with the architectural image; size of building is linked with depth of foundations. A single floor cottage can probably be constructed even on poor ground with very little effort. But a cathedral, with its great height and vast spans, must have deep and solid foundations. With the most successful epic tales you always get the feeling that you are walking into a real world with an existing, if largely untold, history. The reader must sense that were he or she to metaphorically tap the walls, they would ring hard and solid and were they to open the books on the shelves in the heroes library they would have facts and histories in them. This sort of foundational depth is probably not conjured up in weeks or even months. It is salutary to remember that the type example, Lord of the Rings, was fermented and matured for nearly 40 years in Tolkien's brain before it was completed and published.
Thirdly, because of these factors, you ideally need to do what the master did with Lord of the Rings and that is write it all and then go back and edit it. If you write and publish it volume by volume by the time it comes to the last book, with the previous books irrevocably published, you will find that the die is cast. You cannot now introduce major new characters, reveal that your hero is a diabetic or invoke hitherto unsuspected magic or acts of God. Your first pages determine your last.
That sounds like I know what I'm talking about. I'm not sure I do. But I'd wish I read the above a long time ago.