Wondering if there's professional recommendations for the amount of:

1. sheathed cable,
2. unsheathed and unstripped wire(s)
3. exposed copper at the end of the wires.

For:

1. Junction boxes,
2. power points
3. light switch boxes.

NEC requirements are available on the Web.

You can find an explanation, and rules in this web page.

http://ecmweb.com/code-basics/box-fill-calculations

The NEC requirements ksamuiguy posted are interesting but the NEC doesn't apply here.

Conventional wisdom suggests enough slack to re-make the connection twice without making it impossible to remove the fitting. Thai boxes do tend to be rather shallow so take care you don't over pack it.

If using sheathed cable the sheath must enter the box far enough that the conductors can never be exposed outside the box.

Strip enough to ensure the copper goes right into the connector but without leaving any bare copper exposed. Many fittings have a stripping guide moulded into the fitting itself.

Whatever amount is used it is never enough when you have to go back and re-do a connection, known as Somchai's Law

Measure the width of the box your connecting to and multiply by 1.5. That's a workable length

from the point of entry.