I did a poster for a postgrad-only conference in my area (the best I find, because you're all in the same boat and there's less pressure when you can network with someone who has a sense of humopur, rather than a stuffy old professor!) after 6 months, a poster at an international conference after 1 year, then two oral presentations at postgrad-only conferences at the start of my third year, and an oral presentation at an international conference close to the end of 3 years. Good to go to one early, even if you don't present, because its great experience and you'll learn a lot. Funding might be an issue though.
I think if I'd done my big presentation any sooner then my results wouldn't have been quite as good, or my ideas as well formulated. In your 3rd year its scary how quickly things and ideas change! I was also in a better position to talk about jobs, collaborations etc.
I hear about most conferences through societies, so join as many mailing lists in your field as you can find! Try www.conferencealerts.com too, but to be honest i haven't found it as useful.