That comes with personal preference. I can only speak from my experience, but a wine that's been created to last 10 years, and has been aged 10 years, might only require 1-2 hours decanting. I'd just sip it upon opening and every so often afterwards to check progress.. A young wine that's been created to be drunk young might benefit from being put in a decanter, or again, max 1-2 hours. The tricky bit is wines that have been created to be cellared, but are opened young. these will benefit from anywhere between a few hours to 24 hours of being exposed. That said, I wouldn't worry too much about opening them the day before and then going off (provided they aren't exposed to 20+ degree heat). If you're in Sydney or Melbourne or similar, and have a shady bench, just open the Shirazes and Cabs the night before and enjoy a glass before bed. they should still be good drinking next afternoon. If you're a bit cautious, then open at lunchtime of the event.
To stay on topic, I just remembered Vinomofo have a video about various decanting techniques:
https://vinomofo.com/content/videos/vinomofo-tv/vinomofo-tv-ep-10-decanting-wine-673ca5ac