How can I specify time for text titles in iMovie for iOS
If you click on the "?" in the upper right, you'll get yellow markers that show help. One of them is "Learn More" where it brings up the user manual and a search field. When I type "titles" into the search field, it gives me a list of options, the first of which was "Add a title."

According to that help section, a title lasts the entire length of the video or still you apply it to.

So, you can cut your video into 3 pieces - the piece before the title, the piece containing the title, and the piece after the title. To do that:


click on the clip and scroll to the time where you want the title to start 
press the scissors icon at the bottom. This will offer some options.
Click on "Split"


Do the process again where you want the title to end.

Once you've done that, click on the piece you want to contain the title. At the bottom, click on the "T" icon. That will bring up the title selection tool. Choose your title style and click on the text to edit it.