Plan two to six weeks, scaling by signal not by a fixed calendar. Start low — tens of messages a day — and increase only while complaint rates stay near zero and authentication passes. Warm to your own opted-in audience, not a synthetic engagement pool that providers increasingly detect and discount. Watch placement and Postmaster/SNDS throughout; slow down on any reputation dip.
Warmup is about building a reputation gradually so providers learn your domain sends wanted mail. The right pace is governed by signals, not a countdown. Begin with a low daily volume and a high ratio of genuinely engaged recipients, then step volume up only while three things hold: complaint rates stay near zero, authentication keeps passing, and placement stays strong in your seed checks. If reputation dips in Postmaster or SNDS, hold or reduce volume until it recovers. Two to six weeks is typical, but a domain warmed to a genuine, responsive audience stabilizes faster than one inflated with manufactured activity. A caution on "warmup pools": services that generate scripted or incentivized opens, replies, and "mark as not spam" are sending the exact signal mailbox providers now fingerprint and discount — labeling the pool as genuine does not change the mechanism, and it can burn the domain you are trying to protect. Humerly manages the ramp to your own opted-in audience and measures placement as you scale.