If you stopped advertising today, how long would it take for sales to drop to zero?
It’s a good question, and for many household names the answer would likely be ‘never’: they’ve built up enough of a baseline that so long as they have distribution, they’ll continue to pick up sales.
For smaller brands though it depends on where you’re advertising. Some channels have more of a delayed effect than others. For example if you switch off Google Ads today, sales from that channel would be immediately gone. However organic SEO might continue for years depending on how competitive your niche is. As a general rule, the more editorial and emotional the content, the longer the adstock (duration of the effect), so you should expect video ads to last longer than text ads.
MMM is one way to estimate the adstock effect, and it can be helpful to know the rate each channel would drop off. If you’re increasing spend in a channel with a big lag, expect performance to look temporarily terrible, and if you’re cutting spend expect results to look amazing in the short term (before the lag catches up).