Now I know right away that telecom is a part if IT that most peope may not even belive is IT, but thet shape of things in the last 10+ years has certainly changed, and is only moving faster. When I started, everything was TDM, and PC's were just starting to be put on the big guys desks in my company. Since that time, we have almost tripled in size, and everything that was standard then is obsolete now. One big part of this is VoIP and convergence within IT.
Again, when I started, voice and data people did not speak, let alone work in the same group. Now I manage the voice AND the data group, which is one in the same.
So what does all this have to do with your questions?
1) Things can and will change very fast (See Moores' Law)
2) We have no idea how fast they will change
3) We have no idea when they will change radically, not always for the better.
My $.02 would be that if you are getting out of the field for a while, KEEP UP TO DATE! This is not too hard to do. Make sure you know what is out there, so the transition back in will not be so foriegn. There will ALWAYS be "(testing/guess n check)", becasue if not, we not longer provide any value as analytical human beings. Anyone can just install a service pack, especially if there is no need to test and verify it works. If everything just worked, software vendors would make all the money (I guess they already do), but there would be no need for people in IT.
Again, just $.02 from some guy in telecom that loves what he does every day.
Scott M.