Django and units conversion
It depends on how you want to use it. Let's say you have length value and two possible units, cm and mm. If you want only to print the value later, you can always print it as [value]&nbsp;[unit].
However, if you want to do some calculations with the value, for instance, calculate the area, you need to convert the values to the same units. So you have to define unit conversion table anyway.
I would convert the units to the same internal unit before I store them in the database, rather than converting them every time I use them.
I would add a model for units and physical quantities only if there are too many of them and the conversion is really tricky. Such a model could work as a converter. But for simple cases, like mm⇒cm or inch⇒cm, a static conversion table would suffice.
I need to store some values in the database, distance, weight etc.
In my model, I have field that contains quantity of something and IntegerField with choices option, that determines what this quantity means (length, time duration etc). 
Should I create a model for units and physical quantity or should I use IntegerField that contains the type of unit?