I've said it in another thread, but I'll say it again.
I feel the best time is when you want to pre-order a game, and do some trade ins. Preferably trading something with a decent value, and even better if it's during the trade 3 get an extra $10 event.
Example: I traded Yakuza and 2 unimportant useless games in. Yakuza got me around 30, maybe 4 bucks total for the 2 others. So there's 44 already. I wanted to pre-order ffxii, so theres 30% of 44, plus the 10% for signing up. Comes out to making like 18 dollars, which paid for the subscription/card.
If you don't use it often, hold out for something like this.