I don't really look for a particular length other than a usable amount. The length of usable and ripe, meaning sweet, cane very much depends on weather condition, water and fertilizer so sugarcane can be ripe and have many feet of usable cane in just one season if grown in Florida or Hawaii but it can have just half that length and not be as sweet when grown in SoCal.
If you planted your cane from a new section of cane this year, your cane may not be fully sweet because it takes some time for the roots to grow out and get established. But, if your cane is from a patch that was established last year and it had this full year to grow, it can be ripe even if grown here in Cali.
Simon