It is a skill of only four heroes in DotA. These are – Lion (.5/1/1.5/2 secs. stun duration), NerubianAssassin (.75/1.25/1.75/2.25 secs. stun duration), SandKing (Burrowstrike: 1.65 secs. stun time at all levels) and Tidehunter (Ravage: 1.5/1.8/2.25 secs. stun duration).
Apart from this stun duration, there is an “airtime” which units undergo before the actual stun. The duration in the air (airtime) is 0.52 seconds for all types of Impale (Impale, Burrowstrike and Ravage).
So the actual disable time for:
Lion: 1.02/1.52/2.02/2.52 secs.
Nerubian Assassin: 1.27/1.77/2.27/2.77 secs.
Sand King: 2.17 secs.
Tidehunter: 2.02/2.32/2.77 secs.
Also to note, you can attack these units and cast spells at them while they are in air. So you can take these as the actual stun durations effectively.