It tends to vary from one to the other. I did a 2 day course followed by a test on the 3rd day. It cost a small fortune though! I'm glad I did it as now I have absolutely no worries about what I'm towing and know that I've been passed as being competent to do so! Some instructors do 1 hour lessons over a period of time, others do as I did and you do an intensive course. I was lucky that I'd had a 4x4 and a trailer for about 18months before I actually got around to doing my test - just used to tow with a friend (that had the correct licence) and L plates. I think this stood me in good stead and I passed first time. As long as you can master the reversing the rest is down to good driving.
I did the same as s_v, 2 days intensive then test at the end, again, cost a lot of money ( but then everything does round here!) but agree that it was well worth it as I'm now v confident and competant, in fact often end up parking trailers etc for other people!
Shame my OH's new horse was too big for our trailer so I've now had to do my HGV....but on the brightside I'm now a qualified 'trucker'!! And same applies with the lorries, even though lots of people older than us can drive 7.5t on their licence, it was def good for me to do the lessons/test for my lorry licence.