almost never--for me this is important. As I really get very few flats per year, usually about one on average, my tubes can be in use for quite a while. I'd rather not take the chance of a patch failing, what I can say is that I dont ever recall a patch leaking afterwards if put on properly.
OK, if one failed repair in 20 years (my experience) is too much, that's your choice. I almost never repair the tube on the road, I carry a spare tube or two with me and repair the damaged one at home in comfort.