Board Thread:Questions and Answers/@comment-25612229-20141102023251/@comment-1352718-20141103033231

In-Universe explanation is that he usually gets knocked out of his alien form in like a minute because he randomizes the time-out by slamming his Omnitrix too hard. Also it seems that his Omnitrix drains faster as well as shown in Darkstar Rising where after transforming from Echo Echo to Spidermonkey to Jetray to Humungousaur to chain combo Darkstar alongside Kevin, he already was out of his Omnitrix transformation.

The same is shown in Good Copy, Bad Copy where Ben fought Albedo for less than three minutes, constantlyswitching and after transforming into Chromastone for about one-two seconds, he already was reverted out of his transformation.

Real World Explanation is that Ben was extremely overpowered with Quick Changes as seen in Darkstar Rising where he literally destroyed Darkstar with Kevin and Gwen's help which shows the full extent of what quick-changing with the Omnitrix can do, so they just dumbed it down for plot convenience and to give other Aliens more screentime.