Lipo Battery Drain with 3V Regulator
The parameter you should be looking at is called quiescent current (Iq). This is the current consumed by the regulator when the output current is zero. Both linear regulators (e.g., LDO's) and switching regulators (DC-DC converters) can possibly have high Iq. In general it is harder to make a DC-DC with very low Iq.

If you need to leave a regulator connected to a battery at all times, you should be sure to use one with a low Iq. To figure out how long the battery will last, divide capacity in mAh by Iq in mA. That will tell you how long the battery will last.

In your case the Iq is 5mA. For some reason the datasheet uses the symbol Id instead of Iq. But it is the same thing. Anyway, 850 / 5 = 170 hours or 7 days from full charge to flat empty.