the current banking window ends 8 months from the start of your UY.
i have an oct UY, so my 2008 pts are available for use from oct 1, 2008 - sept 30, 2009. if i decide on oct 2, 2008 that i won't be traveling during that period of time, i can call MS and bank my pts into my 2009 UY (oct 1, 2009 - sept 30, 2010). (BTW, they cannot be banked again and will expire at the end of that UY if not used.)
i can make that call any time up to 8 months into my UY (may 31, 2009) and bank my pts into the 2009 UY. BUT if it dawns on me on june 1, 2009 that i'm not going to use my
DVC pts before sept 30, 2009, it's too late. i have to book a reservation prior to sept 30, 2009 and either give it away to family or rent it for cash...or else those pts are lost and gone forever.