I could spend a whole day playing this game in my office and risk getting fired.
It's challenging, deep, requires skills, patients, you need to make quick decisions and be very smart at later levels. The first few levels are quite simple, but when you play further, those simple elements add together and become something really complex and challenging.
Warning, you could spend 30 mins in one game, then ruin it by making a small mistake, get ready to buy a new mouse or keyboard! (Though I honestly think that if you spend that much time in one game, you maybe doing it wrong in the first place.)