What is the maximum light difference for one to be able to see into or out of a building and equally discern objects? In this question I am referring to the physics behind not being able to discern objects outside when in a lit room, even though you would be able to discern them if you were outside, even though when outside you are able to discern objects inside the lit room.
This is the stuff that makes "one way" glass and window tints produce the desired results.
 A: A sheet of normal glass transmits of order 95% of the light incident upon it (assuming normal incidence and clean glass). Conversely, that means that 5% of the incident light is reflected. The exact percentage depends on the (wavelength-dependent) refractive index, any polarisation that the light has, and the angle of incidence. Details can be found by using at the Fresnel equations or by using this applet I designed.
When you look at the pane of glass (from inside or outside) you see a mixture of light that has been transmitted through the glass, or reflected from it (from either the air/glass or glass/air interface). At some threshold ratio, your eye/brain will start to choose the transmitted or reflected image over the other. This cannot be an exact science; there is definitely a grey area where you can "choose" to see both. 
The ratio of reflected to transmitted light will depend on the ratio of light intensities inside and outside the window and the 95/5 figure I quoted above  (hence, it will also depend on angle of incidence, polarisation and wavelength). A simple algorithm could measure the light intensity inside the room and outside the window. In the example given, unless the light inside is 20 times brighter than the light outside, then light transmitted in will dominate over reflected light. On the other hand, if we look from outside, reflected light dominates until the light inside the room exceeds 1/20 of the light intensity outside. 
"One-way" mirrors (which is a misnomer) would normally have some sort of reflective film (aluminium) put on them to reduce the 95/5 ratio, which would alter the discussion above.
A: To a decent approximation, it should just occur when the amount of light hitting the window from the outside is equal to the amount of light hitting it from the inside if there are no special coatings on the glass to enhance reflection one way or another.  In other words, if the amount of light being reflected back at an outside observer is equal to the amount of light being reflected back towards you, you will each have roughly the same visibility of one another.  It is a hard question to derive from first principles with great precision though, because there is a difference in the transmission and reflection properties of glass with different wavelength and incident angle.  These are typically different for indoor light and natural outdoor light.  There is also a difference between a patch in your window that is difficult to see through due to glare and the whole scene outside your window being hard to observe due to low illumination outside and too much back reflection inside.
In practice if you don't have photodetectors set up to continuously monitor brightness to determine when your blinds should be open, you may want to think about an algorithm determined by astronomy.  It is predictable when the sun will rise and set (and its trajectory through the sky if you're interested in the effect of angle as well, i.e. seasonal changes).  You could collect a little data by placing two identical objects at fixed distances (garden gnomes or something) inside and outside your house and seeing how the position of the sun in the sky correlates with your being able to see better inside or outside.  Just remember to keep the lighting in your house constant for all measurements if you're feeling up to the task!   
