Our limit, set by the RE, was 6 months. We're extending it simply because we're not sure yet we want to pay the money for something with such a small success rate.
Due to my AMH levels (Anti Mullerian Hormone, also known as ovarian reserve), it is quite possible that I am a low responder. This means that stimulation, as part of any IVF/AIH/whatever procedure, might not at all work. I could get none, one or twenty million eggs, but of course nobody can tell. That's why I'm not just jumping into IVF, even though I should because, well, I have low AMH levels and my time has pretty much run out.
I do ovulate, I am healthy, I am 34 years old, but my ovarian reserves are those of a (pre)menopausal woman and there's no time to waste. That's why I've had a RE since before I even got off the pill. I learned about this AMH almost by chance (we didn't go to a RE to get either of us tested, but instead we wanted to know what was available in terms of embryo preservation), but the doctor decided to run a quick test to see whether or not we should waste time on embryo preservation to begin with.
On top of that, my husband's SA came back with 12 million instead of more than 15 mil. sperm, so this adds onto the pile of why it is highly unlikely that we'll conceive without assistance (yet assistance might be of no help at all).
Long story short, our chances are a lot lower than those of a healthy couple and the only reason we're not trying IVF are the insanely idiotic prices that come with the American healthcare. Unfortunately I can't get off work either to fly back home and get it all done for next to no money (it's covered by my insurance over there).