If you only want to know ON/OFF duration of 18v signal, why not use the signal to control the base of a transistor that would send 5v to a PIC?

Then I would use interrupts on that pin and use a timer.
I'm a novice when it comes to the timers and interrupts. Seems like a good idea that's worth trying.

1 does the line idle high or low
Idles low.

2 what does a start bit look like
1ms on/1ms off

3 how many bits in a session
100

4 100uS / Off - 400uS a 1 or a 0 ?
It's a 1 but it can vary. Before the packet is received, I will know what is a 0 and what is a 1.