Please leave the captions up longer.
It took some digging to find these sites. Neither one may exactly fit what you need to do, but both can serve as guides.
The basics of on-screen typography: [
jonnyelwyn.co.uk] says you should be able to read the text three times.
The cac Captioning Style Guide: [
www.dcmp.org] says more than I want to cut/paste here.