Adjusting FCPX caption default duration?

No, you can't.

What you can do though is to set a range in the timeline (or clip) and the new caption's duration will match.

As a small addition to spherico's answer:

You can change the duration of multiple captions at the same time.

Select them, press control-D and type a new duration, eg type 5. (the digit five and a period) for five seconds.