Well.... I could say "I fix everything that I touch", but that would be a monstrous lie. I've been at this for over 30 years. I do my best, but a severe ass-kicking is always just around the corner.
When dealing with wiring issues, a hard fix is pretty much known when you find a break, corrosion, worn to ground.... situation.
Intermittent faults are trial and error. Take your best guess and wait it out to see if it repeats or not.
Box changes can also be frustrating. You change a component to fix one problem and another pops up because the unit had other issues.
There is no set time or average for aircraft avionics/electrical repairs. With mechanical problems, they can be seen. With A/E issues..... electrons can't be seen. I have to rely on test equipment to help me out. You take in what you see, decipher it, and take your best shot at it.
I sometimes use the comparison of traveling down a road towards the "fix", only to find I'm driving the wrong direction. You back up and take another road. Sometimes several roads. There are some problems that you never find the right one to take.