Meditating whole day is neither advisable nor possible. Even for yogis who have withdrawn from the world and seek solitary nirvana as their goal, it is impossible. An odd exception here and there of this kind of intense meditation may be due to some past preparation but otherwise it is not possible and also not desirable.
Meditation is primarily a mental process. At best and if truly done with persistence it may take us towards an impersonal peace. At worst it can close us in some limited mental formations which we may take to be final and even ultimate. But most often it has a tendency to become mechanical, a state in which one may sit for hours without anything changing within. Sooner or later the other parts of nature step in with their demands – the body with hunger and sleep, the vital with restless impulses and sting of desires, the mind itself starts wandering into various realms and enters a dream state, swapna samadhi.
An intense concentration upon the Divine with the help of a mantra or japa suffers from the same setback. After some time, there is fatigue and consciousness tends to relax. That is why all those who have walked the way, advise a healthy balance between work or karma yoga and meditation. Works, especially niskama karma and the practice of equanimity helps the mind to enter into deeper states of meditation much more easily. Now a days of course meditation with guided imagery is much into fashion. All these things are temporary in their effects and help us get some kind of a mental and vital peace but they are not the core of spiritual life.
The core of spiritual life is aspiration. It is this towards which one must strive, to increase one’s aspiration and seeking for the Divine. Aspiration is a real need and is much more than a technique. It is a real fire and not an image though one can use an image to start with. This aspiration can continue for far longer periods since it needs an in-gathered concentrated state rather than sitting in any fixed posture and practice any fixed mental discipline or technique. With this practice slowly a general meditative state tends to develop which is a far greater help towards Divine realisation than continuous meditation. Work done with right attitude should be balanced with twice or thrice short periods of meditation where one simply opens oneself to receive the divine or else concentrates on the Divine Presence within the heart or above the head. If one sincerely does this then all the rest develops in due course of time.