Save as much as you can whilst still enjoying what you‘re doing. If this means working on a prototype in your spare time so you can seek investment, then I‘d recommend that.
The biggest expense for most indie developers is living costs. I made the mistake of living in the same flat with the same rent when I was an indie developer as I did when I was department head at a global financial company in central London. I wasted thousands by not moving out sooner.
If you can trim expenses, do so. You might have made a budget that means you only need to make minor reductions in outgoings, but remember that your game might well need longer. You may get a golden opportunity at the end of the dev cycle, or your game may bomb and you‘ll be desperate not to give up. Money is time, and you may need more.
Seek investment. Move somewhere cheaper. Cook your own meals. Start shopping smarter. You have flexibility in time, so use this to save money. If you‘re in the UK then I strongly recommend registering as a sole trader and immediately checking whether you are entitled to Working Tax Credit.
Getting an accountant or learning about tax structures and welfare benefits could save you large amounts of cash. Seek professional advice, and you may be able to claim tax relief on your rent, service bills, purchases and even past purchases.
Seek out grants from local government. You may be able to claim R&D tax relief for writing a novel engine.
Start saving straight away, as you don‘t know how long you might want to last for.