Collaboration, tutorial co-authorship, mentoring, talks, reviewer invitations — all welcome. I try to reply within a few days.
If you're writing about a graduate project or a thesis, send a short paragraph describing your question and the kind of data you have or want. That gives me enough to say something useful in reply.
For collaborators, hiring conversations, students, and tutorial co-authors. Slots auto-update from my Google Calendar — pick anything that's open.