Casual is not typically defined in terms of time. Or at least not time alone
Casual players can be a little time or a lot, casual playing is more meaning that end game goal is not the single intent. Casual players tend to be as much or more focused on social aspects, role playing aspects, exploring for the sake of exploring rather than being primarily or solely focuses on character advancement.
As opposed to hardcore players, who do gear themselves primarily towards progression of power. This player can be on 10 hours a week or 30 - the point is that they aren't the role playing, meandering for the sake of meandering, sit and chat kind of person.. they are focused primarily or solely towards progression in the game.
Being hardcore doesn't mean spending 25+ hours or some arbitrary benchmark of time in the game as a first necessary benchmark - or at least it shouldn't. Those days of gaming mentality started to die a long time ago, and good riddance when they are finally gone forever.
Hardcore is much more about learning the mechanics, gearing to the mechanics, and progressing your character to meet the end game challenges. It is a lot more fun to do that with your friends and "crew" rather than re-levelling to even be able to begin.
And that is the point I hope I am conveying - time sinks for the sake of time sinks on TR's experience curve isn't any challenge, it isn't any benefit to the players or long term for the game.
"Enjoying the trip" is fun the first ten times... but seriously, even Delera's gets a little old after 50 times through it, and it doesn't even have the merit of relevance to end game as grinding at cap would. I would prefer that my time sinks are in some way at least relevant to end game, and allow me to continue to play with my friends at that level. (I prefer the challenges, for instance, at least they are relevant content for my 20th characters... bonking Coyle over the head for the 9 millionth time, however, is not.)