When I availed of thef Rustan's discount card (around 2003) least P50k worth of spending was required. You can collect receiots from various SSi establishments. And the discount card may be used to avail of 10% discount on regular items (not marked as "X" "C") when paying in cash, 5% for card purchases.
It would be ideal if you have a Citibank Rustan's card, you can avail of 5% rebate for Rustan's and SSI purchases.
Does your Rustan's blue discount card have an expiry date? I lost my original card I am not sure if there's an expiry date, when I applied for a replacement card in 2007 meron na and it has to be renewed yearly.