LTS, all I grow are loose leaf varieties, of which Romaine "are one."   Don't worry about ripe.   You can cut them whenever they reach the size you want, and before they bolt.

I rarely harvest a whole head of lettuce. Instead, I start removing outer leaves as I need them as soon as the plant is big and full enough to continue to thrive. Two or three heads of loose leaf lettuce are all I need to keep two of us in lettuce throughout the season, until they bolt from the heat.