It seems that the real answer is "it depends": it is best practice to avoid these inline couplers if at all possible, but you can probably get away with one (or at most two), with enough added precautions thrown in as well (dielectric grease, shrink tube, etc...).
This blog article article on the TrueCable website covers exactly this subject in great detail:
Everything you ever wanted to know on the subject is right there in this article.
This blog article article on the TrueCable website covers exactly this subject in great detail:
Ethernet Couplers - How Many is Too Many?
Ethernet Couplers - How Many is Too Many? Written by Don Schultz, trueCABLE Senior Technical Advisor, Fluke Networks Copper/Fiber CCTT, BICSI INST1, INSTC, INSTF Certified Have you ever found yourself in a situation when you have two short Ethernet patch cables and need to make a single longer...
www.truecable.com
Everything you ever wanted to know on the subject is right there in this article.