Obviously, it can vary widely due to a number of factors but in general eight weeks seems to be the magic number.

Via Daniel Coyle's excellent book The Little Book of Talent: 52 Tips for Improving Your Skills:

When it comes to growing new skills, eight weeks seems to be an important threshold. It’s the length of many top-level training programs around the world, from the Navy SEALs’ physical-conditioning program to the Meadowmount School of Music program to the clinics of the Bolshoi Ballet to the mission training for the Mercury astronauts. A recent study at Massachusetts General Hospital showed that practicing meditation for twenty-seven minutes a day created lasting brain changes in (you guessed it) eight weeks.

Of course, this doesn’t mean that you can be proficient in any skill in eight weeks. Rather, it underlines two more basic points: 1) Constructing and honing neural circuitry takes time, no matter who you are; and 2) Resilience and grit are vital tools, particularly in the early phases of learning. Don’t make judgments too early. Keep at it, even if you don’t feel immediate improvement. Give your talent (that is, your brain) the time it needs to grow.

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I've often kept as a rule of thumb that it takes 60 days to turn something into a habit.

If you can do something every day for 60 days, by the 61st day it becomes a part of you.

Not sure if I read that or just believe it.

Other research shows it varies widely depending on the habit. 60 days is a good average, though, and if it works for you that's all that matters. :)

It works for me as a rule of thumb but it's not a hard set number. )