[pdonis]

> is this spontaneous collapse quite rare in practice ?

It's extremely rare for a simple quantum system like a single electron; but it is happening basically all the time for a very large system like the detector in the double slit experiment. So basically, the electron is virtually certain to get all the way through the double slit experiment without any spontaneous collapse, but as soon as it interacts with the detector screen at the end of the experiment it will have to collapse basically immediately, because the detector screen is always having spontaneous collapse events and the electron is now entangled with the screen and has to collapse along with it.