Sunscreen is a drug. The formula work is not the long pole; the testing is, and it is serial, not parallel. Here is where the weeks actually go.

The map

Phase	Weeks	What is happening
Brief and benchmark	0–1	Target SPF, filter system, texture benchmark, claims
Bench	1–4	First submission, two revision rounds, in-vitro SPF screening
Scale-up batch	4–6	Scaled on the production line, not a beaker
SPF testing	6–12	In-vivo SPF, broad spectrum, water resistance at an outside lab
Stability	4–15	4/8/12 weeks at 40°C, freeze-thaw, packaging compatibility
Labeling review	10–13	Drug facts panel, claims, artwork check
Production	13–17	Batch record, in-process checks, release, fill

The four things that add weeks

In-vitro screening skipped. If you go straight to in-vivo and miss your SPF target, you have burned seven weeks and a full in-vivo panel to learn something an in-vitro screen would have told you in days, at a small fraction of the cost. Screen first, always.

Filter stability. A hybrid system that pairs zinc oxide with bemotrizinol (BEMT) still has to hold its SPF through the in-vivo test and through twelve weeks at 40°C. Get the dispersion or the emulsion wrong and stability fails at week eight, which is after your SPF test has already passed on a formula you can no longer ship.

Packaging compatibility. A sunscreen that is stable in glass can separate, discolour or leach in the tube you have already ordered 20,000 of. Run compatibility on the actual component, at the same time as stability, not after.

Drug facts artwork. The panel has fixed content, fixed order and minimum type sizes. Designers get this wrong constantly, and the correction round lands right when you wanted to print.

What you can compress

Stability and SPF testing overlap. So do scale-up and labeling. A well-run program is twelve weeks because those overlaps are planned in on day one; a badly-run one is twenty-four because each phase waits for the last to finish.

Ask your manufacturer for the Gantt before you sign. If they cannot produce one, they have not run enough of these.