Partial charges in a covalent bond
We describe an object, which does not exist, but is very real, called and 'orbital'.  It is a region of space where an electron is 'likely' to be.  It is the result of charge field interactions between positively charged nuclei.  Normally we only discuss orbitals in the context of  1-2 Angstroms distance between atomic nuclei.  Some orbitals have a character which we call 'bonding'.  A bonding orbital, occupied by one or two electrons might be thought of as a 'bond'.  However, bonds also do not exist, even if they are very real.  

It is somewhat nonsensical to describe where in an orbital the electron is.  If you must, the electron may be described as having a location of the entire orbital simultaneously, with vary weights to all of those locations, which are averaged.  This needs to be thought of in the concept of 'wavefunction', which is a mathematical function describing the distributions of these weights.  In the case of a polar covalent bond, the wavefunction more heavily favors locations near the more electron negative nucleus, and so the orbital 'concentrates' more of its charge near one end of the 'bond' and results in what we call a partial charge.

The reason we get squeamish about describing 'when' and 'where' is that the Heisenberg uncertainty principle renders the uncertainties energy and momentum of the system to be absurdly large when dealing with systems that are roughly the size of a chemical 'bond'.

At this point, nothing should be cleared up with regard to the original question.  

The reality of the situation is that using when and where in this context is very likely to lead to predictions that will lead one astray when predicting physical phenomena.  As bizarre as quantum mechanics is, it has been shown to be highly predictive of the bizarre world of atoms and molecules, and so you have to try to figure out how to not ask questions from a classical  physics framework.