With my first, my mucous plug came out when checking into the hospital (literally, in the urine specimen, which is not good for protein readings!). I was there for around 12 hours before the impatient OBs insisted a c/s was necessary. However, I'd already been in labor for about 24 hours at that point.
I don't remember noticing it with #2, and with #3, it was only a few hours between the start of bloody show and the birth. Though, again, I'd been in labor for hours and hours at that point.
My best recommendation would be to learn the "emotional signposts" of labor. Those seem to be pretty consistent across most births (not all!) and may be your best indication of when to go. You really do want to stay home as long as possible, but I understand that may be tricky with a long drive!