How long should I warm up a new domain before sending in volume?

Plan on roughly 4–6 weeks for a completely new domain with no sending history. Start around 20–30 emails/day in week one, and increase by no more than about 50% week over week from there. Going faster than that is the single most common cause of a new domain getting flagged before it's even had a chance to build reputation. It's slower than most people want, but it's the difference between reliably landing in the inbox long-term versus fighting spam folder placement for months afterward.