In general, CIQ watch faces run with as little impact on battery as possible, and there are things about how a watch face run in general to limit the impact. (how often things can be updated, etc). In general, you can probably expect a bit more of an impact than one of the native ones on the watch itself
There are 3 variables when it comes to impact as I see it.
1) The more things displayed, the more the impact. A watch face with all kinds of graphs and data will have more impact than one that just shows the date and time for example.
2) There is a slight impact if a watch face displays the seconds all the time, as part of the screen gets updated every second. I've been making that an option in mine.
3) Background communications for things like weather data
The thing is, you really need to try a watch face and make the call. Sometimes there are options (like turning off seconds all the time and not using weather data that help), but it really depends on what you want to see. Is seeing weather data worth the trade in battery life, for example.
