Kenji,
This depends on whether you require the output to fully settle or not. You can write the a give DAC as quickly as the digital interface allows you to, but the output may not settle.
If you want the output to settle, the update rate is simply the settling time specification.
Here's a blog post if you'd like to read more about settling time:
http://e2e.ti.com/blogs_/b/analogwire/archive/2013/08/26/dac-essentials-understanding-your-dac-39-s-speed-limit.aspx