When you are imaging a fast moving asteroid or a bright planet you need much shorter times e.g. 30, 10 or in extreme cases 5 seconds or less..

The Standard Exposure times recommended by iTelescope of 60 seconds, 120, 180, 240, 300 600  etc apply predominantly to astro imagers rather than science users, and is meant for newbies who might think that a 10 second exposure will give them a nice color view of M51. For scientific work, you can shoot as short as your needs dictate, as we assume you have a lot better understanding of what you're doing.

You wouldn't even need to shoot new darks (assuming you know how to scale them and apply them, such as in CCD Stack or MaxIM). The Bias is the same, and the dark data is simply the percentage of signal in the longer dark, less the pedestal of the bias.

Most frames you shoot won't even NEED true darks under 30 seconds -- just apply the bias and flat, as most of the noise is in the bias, and there's very little dark signal accumulation in those short frames.

You can shoot as low as 0.1 seconds if you want to, but anything less than that risks shutter shadow (artifacts caused by the fact that the shutter isn't instantaneous, but opens and closes like an iris)

Please keep in mind that exposure times below 60 seconds have special surcharges applied to them to cover the costs of telescope operations.