With a 350 mAh pack and a steady drain of 20 mA, you can work out the run‑time simply by dividing capacity by current:
[ \text{Run‑time (h)} = \frac{350;\text{mAh}}{20;\text{mA}} \approx 17.5;\text{hours} ]
So under a constant 20 mA load the watch would run for roughly 17½ hours (about 0.73 days).