PDU every year or in three years

Dear folks,

I have a confusion to be clarified. Kindly guide me to understand the duration for collecting PDU's. I passed PMP in May 2011. Do I need to earn 20 pdu's every year or can I collect 60 pdu's before 3rd year,  and then submit to PMI ?

Regards,

Zeeshan Qureshi ,PMP