Well you'll want to confirm upgrade space availability before you purchase your ticket, and especially before redeeming points for the cert.
They expire 1yr from issue, however you can also use them on future travel past that date as long as you can confirm the upgrade... ie buy a cert now that expires in Mar 2017, and you can use it to confirm in advance an upgrade for a flight that you have in summer 2017.
Also you don't have to call the SB hotline, you just need the cert and redemption code number and can have DLAssist enter the info for you.