I usually patch after I get home, but it depends on what I've done to the tube. I've had some memorable episodes over the years, where I've totally destroyed the tube -- one particular blowout back in 2000 at 47km/h was memorable. However, if I'm on the road, I'll just change the tube, and save the patching until I'm in my garage. I'll keep the patches and glue with me just incase.
Also, when you're a long time between flats, it's also important to check your spares every so often. When I broke my 15,773km run last time out, my spare tube had been sitting in my saddlebag for some ridiculous amount of time and actually had a nasty crack in it. Needless to say, I was glad I had the patches that day.
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I am clinically insane. I am proud of it.
That is all.