I think the quickest transport time I have ever had was 30 mins. Unless you work in the city here a lot of rural services required long transport times. If you are lucky you can work rural and have a hospital right in town, I know I havent been fortunate enough to have that blessing.
But yes during long transport times all you can really do is re-assess. Every 5 mins? Thats bit of an extreme. If the patient's condition warrants assessing that often then yes but most of the patients I have transported on long transfers like got re-assessed every 15 ish mins or so. Only a handful would have been critical enough to require many sets of vitals over short periods of times but those pts were flown out anyways.