MartinCh,
I'm certain Camp Freddie is right...well, he should be; he's been in this industry far longer than I have. Interestingly, this came up for me quite recently. I was asked to take someone for a flight in a privately owned helicopter, as the owner couldn't do it. Although I have an FI rating, the machine wasn't on a licensed airfield; therefore this couldn't be a trial lesson. And although I have a CPL, I couldn't be paid for it, as it wasn't being done under an AOC ( and I'm too old anyway). Therefore I could only take her up as any ordinary PPL would do - for free. Weird thing, Air Law.
Back on topic...I instructed fulltime for a while, and absolutely loved it. Then we had a hard winter, and I watched all the money I'd earned dwindle. I also I began to see the downsides - unpredictable work and pay, never having weekends off, no social life. I'd turned my hobby into a job, and it was beginning to be just that - a job. And it took me less than 1000 hours actually. But then, I always did get bored quickly!
Finally I switched to part time, and the magic started to come back. I now thoroughly enjoy it again, and if we get rained off or have no students, I have another way of earning a living. If I was younger I'd be planning to move on to commericial flying to get a different challenge, but the CAA in their great wisdom says I'm too old, so flying-wise this is a dead end for me.
I don't really care; I have other things going on in my life. I decided fairly late in life that I wanted to be an instructor, despite people telling me I was mad and would never do it - well, I did, and I enjoyed it.
Of course, everyone's different. If I had my time over again I wouldn't do anything any differently anyway....I had a fantastic few years, and still enjoy what I do. Just don't expect things never to change.