I think you'd be able to learn the Greek alphabet in at most an hour, just by writing it out and saying it repeatedly. Making it stick long term would be a matter of a couple of minutes a day for a week or so, then every few days. It's small and simple enough that flash cards aren't necessary.

For more involved facts, Anki seems great in principle, but in practice I've found it hard to create and maintain the habit of doing the cards.