First what I would do is go find any graphic novelist you can get your hands on and flat out ask them what they did for money when not making actual graphic novels. You might reach out to 20 but only get a response back from 2 or 3 but that would go a long way to figure that out.
A lot of graphic novels are compilations of smaller stories into bigger arcs, so I imagine some of it would simply be doing smaller scale stuff until the interest provided a market for a larger collection. But I would wager that the majority of artists that are known for graphic novels are also doing a lot of other illustration work to fill the gaps (probably also including working on their own product lines).
But I think a more important question would be find out - do most graphic novelists work for a company or are they mostly entrepreneurs? That dramatically changes the conversation because you're at this point either looking for a pathway to get hired to a studio or finding out what it takes to make your own IP.
So, just logically if it's to make your own IP, I think in your position what I would do is start taking stock of what kinds of ideas you have that you might want to build stories in. Get a pretty firm foundation of what you'd like to explore, then start doing it on smaller 1 page or 3-panel spreads and start getting feedback via friends, family, forums, social media, etc.
That'll start building some experience in doing the work and also build the foundation of your process. As you work out styles, techniques and workflow all that stuff will directly apply to any larger project you want to do.
But I think it'll do 2 more important things:
- Get more familiar with the worlds and characters and stories you're interested in actually doing. Like you said, it's a LOT of work, do you better like what you're doing enough not to hate it by the time your done. That'll also weed out some characters and stories you just find to be boring after a while.
- Get enough feedback from people to see what others actually want to see more of. You can't really dive into any kind of big project that no one is interested in. If you for example explore 5 or 6 different ideas and people are clearly resonating with one of them, you've done quite a bit of market research right there to determine what might sell.
During that process, the hope would be that you'd obtain an audience large enough to maybe consider doing something like Kickstarter to finish one of your fleshed out ideas you think would sell and everyone has given enough feedback on to make it exciting for them and they're involved.
One thing does seem sure though - you're probably in for some sleepless nights 
