Cooking Estimated Base Duration Formula

Base duration = 1200 + (50 * N) + 400 * (T - 1)

Where N = number of level tiers up from 1-3, since each level of material can be used for 4 tiers of levels - e.g. sorghum grains and molten meat can be used for 90-93, 93-95, 95-97 or 97-99, each of which have different base durations.

The formula is only an estimate, but it lines up pretty well with what we know, and matches my observations so far (reached Cooking 30 today so I had plenty of testing time).