1) Shoe life depends on the shoe and the runner (racing shoes usually have shorter life, heavier runners often wear out cushioning faster, etc) but a ballpark figure is about 1000km - so maybe around a year for you.
2) I second the recommendation above to to get a proper assessment from a podiatrist, it'll help you avoid injury as well as avoiding inappropriate shoe purchases so it's worth every penny! Peak Podiatry are one of the best in Perth for runners, if they can squeeze you in. They have treadmill cameras so can actually show you in slow-mo what your foot is doing each step.
Presumably the flat feet are causing overpronation (ankle rolling in too much as you step). If so it's pretty common, I'm in the same boat. I too run with off-the-shelf Orthaheels for the modest amount of arch support they provide, but the pod identified the need for a little more medial support and just modified the Orthaheels accordingly (i.e added some extra foam).
It often takes a while to get used to orthotics because, due to gait changes, some muscles get used more/less. As always just listen to your body.
With the right orthotics you can pretty much go for whatever shoe you find most comfortable - or get the pod to recommend a few options.
Edit: Thought I should clarify the final sentence.. Many shoes have pronation control which may lessen the need for the orthotic to compensate for it. Usually you'll just be shopping for a neutral shoe if you have proper orthotics.. but again, your pod knows best