I can now snooze messages until "someday" but when does that day arrive? I'm afraid that it will either be a random day in the future or some other queue for me to (forget to) triage and neglect.
Short answer: Never. It is just a queue that you'll need to review, eventually.
Quoted from Inbox Support Page
Choose the Someday option under Snooze for emails or reminders that you might want to get back to, but aren't sure when. You can go back through your Someday list later and decide what to do with them.
You can see everything in your Someday list by going to the main menu Main menu and choosing Snoozed. The Someday section is toward the bottom.
Your email or reminder will stay in Snoozed until you move it yourself. You can mark it done, move it to your inbox with pin, or choose a different snooze option.