claude_210's advice is okay... but Motion tends to remember the time you put in that new project dialog, which annoys me because I start a *lot* of projects and want a standard time there so I don't have to deal with it.
My preferred method of changing the time is importing clips, selecting the layer, then switching the playhead clock to "project time" and changing the time from there. You have two methods you can use: double click the playhead clock and type in the time (like you would in the Project Settings inspector) or you can click and drag on the playhead clock up or down to change the time. You can watch the layer in the timeline to see its end as you change the project time.
Notice at the very beginning of the gif, I click on the Clock icon, it changes from about 12:07 in blue to 12:15 in white - that's the difference in Project (playhead) Time and Project Length. Set it to length and drag on the clock (frames section for finest resolution). Watch the timeline for the layer's length. At the end, I'm dragging out the Play End marker to the end of the timeline. Click the Clock icon again to go back to the playhead clock. [I didn't do it here, but you generally want to make sure that the end of the layer timeline indicator lines up with the end of the timeline, then set the Project Out marker to the end of the timeline (if it's not already there). The clip I used was exactly 10.00 seconds to begin with.]
Using the Inspector or using this method, it doesn't matter what the New Project dialog time is set for and it can remain at a "standard" template starting time.
