I made my own using bulk materials as well.  I found out what the maximum length of the cable was, but have forgotten it.  I believe it was 24" because that's how long I made mine.  They work fine at that length.  However, (because KNC sucks) we have no way of knowing how much data is being pushed across these cables, and how fast.  The general rule of thumb is that the maximum cable length halves every time the speed is doubled.

Also, capacitance is the issue for the UART chips, not voltage drop, so 28AWG wires are perfect.  You won't get longer cable length by increasing the wire size.

Anyhow, that was a long way of saying these cables can be made at least 24" long, and they work.  I know because I'm using them.