How much mass is the threshold after which quantum mechanics no longer applies in the double slit experiment? The quantum mechanical effects in the double slit experiment was first demonstrated with photons, then also with electrons, then with atoms, then more recently with molecules having up to 114 atoms.
http://www.livescience.com/19268-quantum-double-slit-experiment-largest-molecules.html
Without using Bose-Einstein condensates where we have quantum effects at the macro scale close to absolute zero temperature,  how large can the mass of the particles being tested be theoretically before the quantum effect wave like interference patterns disappear and the results revert back to classical physics for the double slit experiment?
I know this is inversely proportional to level of quantum de-coherence which increases with temperature.  This is for standard temperature.
 A: A simple rule of thumb is given by using the de Broglie wavelength of the particle if it is much larger than the separation  and the width of the slits , or much smaller , no interference will be seen as with simple classical interference patterns. Calculations can be made in the calculator here, for the distant screen assumption:

It is seen above that a number of parameters enter for calculating interference patterns, and thus observations will depend on the setup.
The sensitivity of the detector of course will play a role whether the interference will register. It does depend on the mass but also a number of other parameters whether interference is observed or not.
A: If you don't subscribe to the one particular version of the Copenhagen interpretation that says that at some sort of cut-of scale physics becomes classical again, then there is in principle not a maximal scale where the double-slit experiment stops working given that you can keep it sufficiently isolated from the rest of the universe.
For a look at what it would take to do a double slit experiment with objects the size of cats see: AskAMathematician.
