There is no standard practice, but there is a general idea of how to carve intros and outros in order to help the song be easily mixed with others. They tend to be very simple, with little or no harmonic elements present, of at least 4 bars (around 30 secs at around 120 bpm). Tipically elements are added little by little until the whole idea is constructed.
Here's Nathan Fake's Outhouse as example:
First 4 bars (first 30 seconds) of just percussions, and the kick has little low end. Then a very filtered bass is added, which stays in one note, and that idea continues for another 4 bars while the bass gains more and more harmonics. At the one minute mark (after 8 bars) the full kick (with the low end) and a snare is added, then the hats at the 1:15 mark, and finally the whole melodic idea starts at the 1:30 mark after a small breakdown.
In this case we have a very well-defined intro of 12 bars, or around 1:30 minutes.
The outro, that I'm arbitrarily labeling based on when the bass stops having melodic movement, starts at 8:45 and continues for 2 minutes, where elements disappear little by little until only a synth melody remains.
You can see that there is a general idea of starting simple and ending simple through small increments and reductions, but there's nothing written in stone. A key idea of this example is that the song didn't start with harmony, but it did end in nothing but harmony. You can mix the intro with whatever, but you have to be careful with the outro and select a song to mix with it that doesn't clash harmonically.
Contrary to what a comment here suggests, implementing both beat match and harmony match in your mix is not "hard to impossible" to do. DJs from all the skill range (including beginners) use both all the time, so I'm not sure where that perception came from. Harmony mixing is a very simple concept, so don't shy away from it!
In the end, it comes down to preference. Some DJs find large intros and outros useful, others prefer to go direct to the "interesting part". James Holden is from the later taste (can't find the exact quote or link at the moment), and barely makes use of the outro we just heard in his Balance DJ set at the 48m 27s mark:
So, if you want to make everyone happy, include incremental intros and decremental outros, which is all there is for common practices in intros and outros. Some DJs might not make use of them, but the ones that do will appreciate it. There are many great songs out there without defined intros and outros though.