How to change duration of still clips efficiently

I have a long movie with hundreds of jpeg stills mixed in.
They came in as 10 second clips. I'd like to change them all to 5 seconds.
It's very slow doing this by hand in the timeline.
Is there a better way?
ChatGPT said copy one clip, option-paste to another and the duration will be copied, but it isn't.

Select all the clips in the storyline. Double click the playhead clock and type 5. (five point) followed by return. They should all retime to 5 seconds.

Typically you would enter the timecode of the length of the clip, for example 5.09 for 5 seconds and 9 frames. However, FCP lets you shortcut by leaving out trailing zeroes, if that's all there is. The decimal is not required if you're filling in something like 5 seconds and 24 frames for which you can simply type 524 (followed by return).

sc_fox wrote:

Select all the clips in the storyline. …

Select all the jpgs in the Timeline Index …

Head that problem off at the pass before you begin...

One way to do this would be to export an XML file (v1.9 not v1.10 for recent versions of FCP) and open it in a text editor. You could locate the clips with the duration you'd like to change and make the changes. Then, import the modified XML file.

FCP writes durations in XML files as a ratio. For a 29.97 fps Project, a clip of ten seconds length shows up as 

duration="1201200/120000s"

in the XML file. You could note the name of one of the ten second clips, then search for that name in the XML file and double-check the duration. Then, it's a matter of searching for the other clips of the same duration and changing the duration values.

I'll stop here, as this is a bit "advanced" for many people...

I did that and readded the stills but they still came in as 10 seconds. 
I can’t deal with this reply mechanism on my iphone. How can I eliminate it?

It's a preference setting:

Thank you, this method looks very appealing to me as a former software guy.
I didn't realize importing XML would make changes!
I wonder what other things can be done similarly.

I can't come up with a way to explain this any more simple than three steps but this is how I change still durations...
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This job was 35mm slides with most having notations on the cardboard frames. All stills (main story line) and notations (secondary storyline) are timed at four seconds. I select a few images on the secondary storyline...
 
Double click the timecode in the viewer window and put in 7 seconds as a new duration...

Then those and ONLY those selected images will now time out at 7 seconds, botching the delicate overlay timing already achieved.


You can set the default still duration in FCP preferences in advance of import. Selecting the targets in the INDEX pane, double click the timecode to change duration (45 seconds in this example) and do things that way but that introduces another level of cumbersome into the mix.