Under the 'old' UA program, layovers when using an SWU had to be 12 hours or less. The current rule, according to the agent I spoke with last night, is "within 24 hours". Also, be careful of the fare classes if you are booking online. For some flights (probably international ones) fare classes T,L,K,G & N don't count for Economy to Business upgrades. When making my reservation for KIX<>IAH the online program did a dirty on me, giving my "M" going but "L" coming back. It took 30 minutes on the phone with a discussion with the supervisor and then their fare class desk to decide that I could upgrade to a qualifying "W" fare for an additional $209.