I can't speak for anyone else, but the way I practice scales goes something like this..

When I learned my first scale I learned the A minor scale.. I learned the first position the usual way of repeating the pattern over and over... I just kept noodling with it until eventually I knew it all over the neck. So the A Minor scale has kind of become my 'foundation' scale.. I was jamming A Minor for months, so I got it down to that point where you can just point out random notes on the fretboard and tell if they are in the scale or not.

For example, when learning A Harmonic minor I can view it as the A Minor with a little adjustment, same with Melodic Minor.
I got A Melodic minor down all over the neck in about 30 minutes since I already knew the A Natural minor so well. Just a little adjustment and then memorizing where the adjustments are and then I got it! But you were aksing about keys..

So once I know the pattern all over the neck in one key, I'll just make a backing track for let's say G minor... At first I approach it as "A Minor a whole step down" but after a while it becomes it's own scale and then I work towards getting it down as good as my foundation A minor scale where I can just look at the guitar and see all the notes 'light up' on the neck... Basicly just jamming with the scale and gaining more and more control as you do it. You'll also get the other benefits of actually playing along with something... Phrasing, dynamics, timing and the overall feeling of having fun by just 'jamming out'

I'm not claiming to be able to play in all keys effortlessly, but that's how I'm training myself to do it it and it seems to be working fine for me so far.

I hope my message here was of some help or interest.. If you don't have the equipment to make backing tracks I could help you out.. I make new backing tracks for myself regularly and sending them via e-mail wouldn't be an issue

By the way, I just joined the forum, looking forward to discussing scales, chords, guitars, music and all that with the people here!