What worked for me was going through a series of chords (major, minor, dominant, augmented, diminished, etc) and play them up and down chromatically.
If I find a chord I like, I'll learn to play it in every key.
I find this to be easier than the much appraised circle of 4ths..so that's how I quickly get a chord I like into my system in all the keys..It's the inversions that take time.
The pro's have spoken, now I can give my beginner's opinion:
A key is as bad as you want it to be...I've never struggled with B (so far..& don't intend to!
)..I can remember late last year when I was given that rare chance to play at church..I could only play a bit in F#..I remember accidentally going into the key of B whilst the song was in F#
Hitting the E felt "weird" because I was used to the F.
But with some months' work, I can say that it's just like U13 said...I can play the commonly played songs here with relative ease in all the keys..I admit that a key I've been lagging in of late is F..hence the reason I practice in it more than the others.
But I make sure that in a day, I have practiced at least once in all the keys..
It really wouldn't take 10years to master all the keys, it's more based on the hours & methods you create for yourself to practice..
I spend a handful of hours a week practicing in them all,so I have managed to get reasonably comfortable with all of them in a time most people would consider "too quick"..but it's really not magic..it's doing what I ought to do & with familiarity comes comfort.
Of course I still have some way to go before I can play "all that sweet stuff"..
Ok, nuff bout me...
tip:
If you don't have time to practice in all keys (or you think you don't), instead of playing a 3 minute song in 1 key, pick a certain 10 second verse & play it in all the keys..
Alternatively, you can modulate by half steps at certain intervals to ensure that you do each part of the song in a different key until you've covered the song...Of course more time/ longer parts should be invested in keys that you are uncomfortable with.
Dwelling more on the keys you already master & depriving the keys you dislike only serves as entertainment for you & your audience if any, but doesn't improve much on your shortfalls..
Ok, I know you all know this, but I just had to say it