What is the value in a regular vipassana meditation practice (e.g. almost daily, some days twice, typically 1hr+ per session) if, over the long haul (months), annicca, dukkha and annatta are not realized? Is the absence of these three very Buddhist characteristics an indication that the practice is, essentially, a secular practice with little-to-no spiritual value?
We practice meditation to purify our minds of greed, hatred, and delusion. We can't do this by yearning for ourselves to reach a particular stage of realization nor by yearning for special experiences within meditation.
We purify our minds of greed, hatred, and delusion by seeings these tendencies within ourselves and by seeing the objects of our greed, hatred, and delusion for what they are and becoming bored with them. When we become bored with them; we stop clinging to them. When we stop clinging altogether; we'll reach Nibbana.
This doesn't take months; it takes lifetimes. Who knows how many lifetimes we've already had to reach this point of being interested in bhavana at all? To be a human interested in the development of one's mind is very precious. You don't have to look far to see humans who are only interested in strengthening and beautifying the body, or accumulating money or possessions, or reputation or power, etc. But to be a human interested in the development of your mind, is an extraordinary opportunity.
Yes, it's valuable to practice your meditation regularly even when you don't "see" the progress. Venerable Yuttadhammo recently gave an especially encouraging dhamma talk regarding when you feel like "nothing is happening", in your meditation; it's actually bringing tremendous benefit. I will look for the dhamma talk and add it to this answer if possible. Be well. :)
This 10 minute long dhamma talk by Venerable Yuttadhammo has some very encouraging words for this situation. He is reflecting on the July 20 passage from http://www.buddhanet.net/pdf_file/words_of_buddha.pdf.
If a meditator is practicing diligently and with skillful effort the meditator will surely reap the fruits of insight meditational practice. What are these fruits? The 16 stages of Insight and eventually Nibbana.
As with other skills if one is practicing the technique correctly, diligently and persistently one will advance in the practice, method or technique.
In vipassana one trains to see reality for what it is, i.e. the true nature of reality free from concepts and superimpositions. One trains the mind to stay in the present, to stay with phenomena as they are without extrapolating on them, without liking or disliking them. One keeps the mind at a level of "bare attention", i.e. the objective and non-interfering approach to phenomena. One trains to be with phenomena as they are when they are.
When interfering with phenomena one will obscure and mask their true nature.
By not interfering with them and simply being objective towards them they will at some point begin to reveal their true nature for the meditator, i.e. impermanence, unsatisfactoriness and the ungovernable nature.
In the absence of any insights one must assess and adjust one's practice and find out what causes one to not progress. Usually that is The Five Hindrances which are obstacles to meditative progress.
Also there are The Ten Imperfections of Insight (vipassanupakilesas). The Visuddhimagga, Chapter XX, p. 660-665, goes into detail about what these imperfections are.
Ven. Yuttadhammo has also made a video about these imperfections called "Monk Radio: The Ten Imperfections of Insight (Vipassana Upakilesa)".
The imperfections are not in themselves hindrances but they can become obstacles if the meditator clings to them or regards them as being important instead of continuing to note mental and physical phenomena.
Hope this helps. If you have questions to anything i wrote let me know.