Time tells you nothing about moisture
Evacuating "for 15 minutes' is meaningless — the same pump on a clean system versus a wet one gives completely different results. The only reliable indicator is the vacuum level measured in microns, not pump runtime.
Target: below 500 microns
For most AC installations the target vacuum is below 500 microns (µmHg). Below this, water boils off and can be removed; above it, moisture stays liquid and destroys the oil and compressor.
The decay test
Once the vacuum is reached, close the valve and isolate the pump. If pressure rises quickly above ~1000 microns, there is moisture or a leak — keep drying or hunt the leak.
Best practice
Use short, large-bore hoses and 1/4" or larger fittings, connect the micron gauge as far from the pump as possible, and change oil when it goes cloudy. A digital vacuum gauge (e.g. Fieldpiece SVG3) shows real drying progress.