A casual google search will tell you that the "three weeks to start a new habit" is pretty much BS--it varies person to person and breaking a habit can take even longer, if not be a lifetime struggle.
That said, if you're trying to start a new habit, trying it out for three weeks isn't really a bad way to go, if only to include it in your schedule and built it into the rhythm of your life. And maybe it won't become a habit, but it certainly will be easier to do than before trying it!
I started thinking about this as I'd fallen into the bad habit of not brushing my teeth as often as I should. I know it's gross not to and I know brushing is good for my teeth, but when you work from home, laziness can happen. So this month I've set myself up to brush my teeth at least twice a day for the month. Every time I do, I put a sticker in my calendar. And if I get 30 stickers, I'm going to reward myself. Hopefully having done this for a month will mean that I'll continue to do it for the months that follow.
Next month, I'll have a new goal and follow the same system.
And maybe by January I'll have fallen out of my first habit and need to redo it...and maybe I'll push it for two months to make sure it sticks! It's like doing new year's resolutions, but only one and at the beginning of every month!
We'll see how it goes for the last three months of 2016, and that will decide if I continue doing this for 2017. Either way, I think trying to better myself in these little ways will definitely be a good change!
Oddly enough, I'm struggling to think of what my monthly "rewards" can be--I don't want it to be too crazy/expensive, but I also want it to feel special rather than just something I could do anyway. Thoughts are welcome :)