Quantum Decoherence If I understand correctly, a quantum coherence is when two wave functions with same phase, frequency and period superimpose to become peaks or troughs. 
In macroscopic scale, say a chair is there because by the time you saw it (measured) the wave functions collapsed and they don't longer cohere therefore leaving the object there. This happens in $10^{-31}$ seconds to be exact. 
Basically, when we observe, wave functions decohere super fast, leaving us only one object to be seen. Is this a correct assumption?
 A: Let me clarify a couple of misconceptions in the question - perhaps this will be enough to answer it:


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*Coherence does not mean the same phase: it means that the two waves can interfere. Depending on the phase they may interfere constructively or destructively. We could take as an example the conventional light sources, such as light bulbs: they emit lots of waves with completely random phases, some of which interfere constructively, others destructively, and we observe some average. The quantum light sources, such as lasers and masers, emit coherent radiation, which explains their wonderful properties in terms of observing interference patterns, low beam divergence, transmitting energy on large distances, etc.

*Interfering wave functions correspond to the states of the same object. Decoherence in this context means that we will observe only one of these states. The double-slit experiment is a good example: by trying to detect through which slit the light has passed the observer causes decoherence and destroys the interference pattern.

