the value of a regular vipassana meditation practice 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. :)

added

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.