It's 24 hours, but Verizon has no idea on this, so don't really pay that any attention. This takes 24 hours if you don't turn off iMessage before turning your phone in. It can take upwards of 24 hours, as it takes a little bit to work it's way through all their systems. In all honesty, I think Hangouts would take about the same time, but as Hangouts is available on iPhone for people who for whatever reason go Android -> iPhone, it doesn't matter as much if they can turn off Hangouts.
Also, have your friends turn on "Send as SMS" in Settings -> iMessage, as well as making sure they are texting your *phone number* not your AppleID, and that your phone number in their contact list is listed as a Phone not an iPhone. This will help it take less than 24 hours, otherwise, it's just 24 hours.
That is honestly surprising. Just tell them you need them to "revoke certificates", took me all of 5 minutes...