If I should get a green card in the lottery, how long can I stay in my home country before moving to the US, before the green card becomes invalid?
Once you get your visa, it is valid for six months. Visas will only be issued during the financial year (October-September period) for which the lottery was held. Once the visa expires, if you didn't enter the US and convert it to a green card (i.e.: got a stamp on it at the port of entry) - it expires, and your win vanishes, you will no longer be qualified for the green card you've won.