If it's exponential, we can't extrapolate from one data point. Two grenades could be any t > 12 hours, even 24 hours or far less - but three grenades will be t^2/(12 hours), and four will be t^3/(144 hours^2), and five t^4/(1728 hours^3), and so on. That manual was sloppily written.
Of course, I assume it doesn't matter now, since in classic literal genie vein, I expect it's now "fixed" as in "permanent."
If it was exponential, a graph could be formed where, for example, 2^n. At 2^n=12, n would be the, Let's call it, the "Grenade multitude constant. Doubling it would give the time for two grenades to wear off, and so on, and so forth.
Actually if the time is added exponentially then two grenades would cause the time to be 12^2 hours which is 6 days. Just so you know here are a few numbers for the times. 1 Grenade=12 hours, 2 Grenades=6 days, 3 Grenades=72 days, 4 Grenades=2.367 years, 5 Grenades=28.405 years, and 6 Grenades=340.866 years.
now you look more feminene best wait for this to wear off before your stuck as girls for the rest of your lives estragen slowly courupting your male minds untill you become a girl mentally and get druged at a bar and end up pregnent then having your future kid getting dragged trough time cousing space time anomiles that allthough present and oupertunity to stop any of this from ever happening you cant erase your kid present and from the future from egsistince
Of course, I assume it doesn't matter now, since in classic literal genie vein, I expect it's now "fixed" as in "permanent."