It really depends on the person how fast a permcath fails. It has to do with how well your body heals it self. A fibrin sheath forms around the end of the catheter. The way Belinda (my vascular access nurse at my hosp.) described it to me is that as you know a permcath is a foreign object in your body and the way the body deals with it is trying to push it out and if it can't it forms a sheath around the object, kind of like a "hard sock", her words not mine". I form sheaths relatively fast, where as a woman in my unit had her permcath work for a year and a half before it needed to be changed. Signs like others have said are lots of alarms, low flow rates, and low clearance rates.
For sure before I had my fistula I went through three permcaths in about six months. The first one developed a fibrin sheath like you described, the second permcath just never worked well from when it was first installed. Without even doing the checks for clearance (in my experience) a bad catheter can cause all sorts of problem, high venous and arterial pressures, air in blood alarms and TMP alarms.