Without going into details, I can tell you for many reasons that my core is pathetic in terms of muscle endurance. My core is quite strong from weight lifting, hyperextensions and certain ab "lifts" that I do. But that's not the practical stuff that you need for everyday life. I did 6 minutes yesterday and want to keep building up to 15 and eventually 15 of more intense stuff.
You may be interested in reading this excellent contrarian
view of specific "core" training. Here's an excerpt along with the link to the complete article (Note: CS=core stability; CLBP=chronic lower-back pain):
"It has been shown that during CS exercise, the maximal voluntary contraction (MVC) of the 'core muscles' is well below the level required for muscle hypertrophy and is therefore unlikely to provide strength gains [54-56]. Furthermore, in a study of fatigue in CLBP, four weeks of stabilisation exercise failed to show any significant improvement in muscle endurance . A recent study has demonstrated that as much as 70% MVC is needed to promote strength gains in abdominal muscle . It is unlikely that during CS exercise abdominal muscle would reach this force level ." (http://baye.com/the-myth-of-core-stability/
My own view is that if you are regularly doing intense abdominal exercises (e.g., weighted crunches or sit-ups), lumbar exercises (e.g., deadlifts or lower back machine) and external oblique exercises (e.g., weighted side-bends or machine rotary exercise) with an appropriate level of resistance (65-70% 1RM) and time-under-load (45-90 seconds) your "core" will get about as strong and fit as your genetics will allow. Static holds (e.g., planks) or similar exercises that don't provide any progressive
resistance are unnecessary and largely a waste of time and effort.