You can retire the stopwatch: there is no clinical-trial support for waiting between layers for absorption's sake. The idea sounds plausible — let each product "sink in" before the next — but no study demonstrates better outcomes from spaced application, and the practical evidence points the other way: dermatology guidance cares about the order of steps, not the gaps between them (https://www.aad.org/public/everyday-care/skin-care-basics/care/apply-skin-care-certain-order). Thin, water-based products before thicker creams, sunscreen last in the morning — that's the load-bearing structure. Back-to-back application within that order is fine, and a routine that takes four minutes instead of an hour is a routine you'll actually keep doing, which matters more than any layering ritual.
There is exactly one wait with a real rationale, and it isn't about absorption: applying a retinoid to fully dry skin. The AAD notes waiting 20–30 minutes after washing before a retinoid — because damp skin increases penetration and therefore irritation, not because the retinoid works better late (https://www.aad.org/public/everyday-care/skin-care-basics/care/skin-care-in-your-20s). If your skin handles your retinoid comfortably applied sooner, even that wait is negotiable.
And if the worry behind the waiting is "will my moisturizer dilute my active?" — the best recent data says no: layering moisturizer before or after a retinoid (the "open sandwich") doesn't reduce its measured bioactivity (https://www.jaad.org/article/S0190-9622(25)02062-6/abstract).
So: right order, reasonable amounts, no timer. Spend the recovered 40 minutes on literally anything else.