IBM hasn’t announced a date for this yet. Based on past schedules, a release is usually supported for four to five years from its general availability date, or about a year from its end of marketing date. It depends too, on how frequent the releases are. Since V5R4 came out in February 2006, and its end of marketing date is January 2010, I’d guesstimate support won’t end until some time in 2011 or maybe even 2012.
See this life cycle of past releases for more information.