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. Not worth the possibility of it given how much I commute, and it would be frustrating to lose the time if it was a glue failure, especially if I am running late or just on time. So although I dont like to waste stuff, I think I will stick with "vulcanizing rubber cement". Most likely I will just try to find small tubes sold separately, or just buy another $3 patch kit. I always make sure there is one unopened tube in my little plastic box repair kit, two new ones if I am on a trip.
I do like the idea of finding a larger tube of vulcanizing stuff to have at home so as not to open the weeny ones.

thanks for your comments, it is good to know that the Elmers stuff will work very well, I appreciate that if you have used it for years with hardly any failures, that certainly is a good track record. Good to know especially if for some reason this was the other stuff available-I suspect your good technique of cleaning the area of the tube is a factor, that seems to be my experience with patching in general.