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Film according to my instructor is still useable after 4-5 stops of over or under exposure depending on certain factors...
Referring to colour negative film, I agree about the overexposure latitude (see my post above), but not the underexposure latitude - if you apply the same criteria for 'usability' then colour negative film will always have a lot more overexposure latitude than underexposure latitude. Apart from anything else, the amount of latitude will depend on what 'usable' and 'latitude' mean to you. There is a standard definition for quantifying and comparing 'latitude', but where would we be if everyone used standard definitions? If your definition of 'latitude' is based on a grainy rendition of the highlights with few or no midtones, then you might be able to stretch to four or five stops of underexposure.
The ISO standard for the speed of colour negative film is based on having one stop of underexposure latitude (according to their definition of latitude), and that is borne out by practical experience. It might be a little more, it might be a little less because of the variety of factors that affect it, but it is very unlikely to be much over two stops even with diffuse lighting and a narrow subject brightness range.
In practice the graininess of colour negative film increases with less exposure - dramatically so with some films. If you underexpose you will get more graininess. The opposite is true - if you give more exposure you will get less graininess. This is a useful feature. As you expose even more you will begin to lose some definition - ie the image will look a little softer, and noticeably smoother. This is another feature that you can use to your advantage in some situations.
Scanning the film and post-processing digitally makes it easier to get good results from greatly overexposed colour negative film than it would be from optically (wet) printed film. This is because of what is known as colour crossover - which is correctable digitally, but very difficult to correct with purely optical/wet printing.
By the way, the exposure-graininess relationship I mentioned above holds for colour negative and dye-image B&W film only. It is usually the other way round for silver-image B&W film: the more exposure the greater the graininess.
Best,
Helen