I would think the quality of the sound being played into them matters to the degree of; if the source can't reproduce the all frequencies to move the diaphragm fully then you won't be doing much stretching (trusting it actually works). Unless you're using really low bitrate sound that doesn't cover the frequency spectrum or your source is something low-end like a headphone out on a cheap set of logitech gaming speakers, then it should be fine.
http://www.burninwave.com/ has a pink noise generator and an app that lets you specify the burn in time (4 hours noise, 30 mins cool off or whatever you prefer). I burned my last pair in by listening to them normally and leaving them on my chair playing my music a little louder than normal when I wasn't using them. I have no idea if burning them in did anything but I convinced myself it worked because I like to get free audio quality for nothing plus the thought of making the driver's diaphragm stretch it's wings a bit sounds beneficial. I'm not burning in my set of HD 600s though because I consider it a waste of time since they sound great already and will arrive there on their own if there is a there. If I don't notice a difference then I'll burn them in until I believe I do. It doesn't bother me either which way.
As for analog/digital, I have no idea if analog/digital make a difference, if the difference between those two did make a difference for burning in headphones, I don't think it'd be possible to measure it but a 0.000001% can feel like a 10% difference if you imagine really good. I don't know if the driver's responses flatten over time, I'd think they do a little over years but that's based on me speculating. I only saw one test where someone compared raw phones to burnt in phones, their test didn't detect a difference in response, but they only burned them in for 5 hours (some ppl know this test), if the test had compared raw phones to a 4 year old pair they may have seen something.
As for the burning in length I would say to burn them in until the sound is good for you. If you're a skeptic then you could be waiting forever but if you're a believer then 40 hours of burn in plus 3 months of usage could be enough (was enough for me) or 300 hours could be enough. I wish there were some scientific studies on this I could reference but I'm not aware of any which prove or disprove it.
