It takes me around 2-3hrs of Mahasi noting to reach dissolution nana. Blackness behind my eyes starts to strobe (sometimes it shakes) just like my vision with eyes open, after few hours of meditation. I'd classify my experience now as 'early dissolution'. What constitutes mature dissolution? What's the criteria? 

Being in nana X means iterating through nanas 1..X-1,X during a sit or does a sit starts with X? Or maybe X becomes the default state even outside of meditation?

Paul:

Whenever you are using some type of meditation technique, there will be cycling, whether on cushion or off.  If you do not meditate off-cushion, than you will still be cycling, but more slowly.  This slow speed will tend to cause you to start in the first nana in a new sit, but will not always be the case (i.e. in some sits you might still start in any nana between 1 & X).  
It might feel like you stay in your cutting edge all the time, but really you are still cycling.  What is happening is that your cutting edge leaves a sort of energetic imprint on your mind (or afterglow) that lasts despite being in other nanas throughout the day.  

For me it just felt reallllyyy tired, reallllyyy lazy.  Don't want to go to work.  Just want to get sucked into the quicksand.  Let myself deflate.  But I kept noting and noting and noting anyway.  And then BOOM.  Fear, baby!

Noah:

I sometimes observe ignoring impulses do do stuff. I notice an impulse: "do X", but immediately I think: there's no need to do that now. Is that what you mean by laziness?

What was Fear like for you?

Paul:

Yeah.  But its also more subtle or widespread.  I've felt dissolution (especially the first few times), as a vibe throughout my whole being, a certain uncomfortable sucking downwards into a hole, spread throughout my limbs, etc.  It might not be that extreme, or take that specific form, but in general it will be felt in the body, in addition to mind patterns.

Wowza- just reliving this experience.  As a cutting edge, Fear felt like knives in my body (and mind), pulling me down.  The entire Dukkha nana spectrum was very much felt in the body for me.  As an aside, here is a synchronicity related to my first experience of that:  

The first time I recognized getting into the 6th nana, I was leaving work (a new job that was already giving me anxiety).  It was pouring rain, completely dark outside, and I got a flat tire in the middle of the New Jersey turnpike.  I knew how to change a tire but had never done it myself before.  The only thing I could think was "Welcome to the dark night."   

Sounds unpleasant.  I observe energetic phenomena in a hypnagogic state between being awake and sleeping when I wait to fall asleep in my bed. Sometimes it's pain, sometimes it's ecstatic energy moving through my stomach, strange dreams. Interesting stuff.

Regarding synchronicity: I see a lot of repeated numbers when I look at the clock, for example 11:11, 14:41, 20:20 - low probability events occuring too often. Strange stuff

The hypnagogic phenomena and clock synchronicities are very interesting.  Hopefully you will experience the nanas through energetic or trippy lenses (i.e. the ecstatic energy) and not purely in negative ways.  I know other people that had this type of passage.

I had a tough time with fear at one point early in my practice. When I closed my eyes to meditate, it felt like I was looking into a deep dark ocean and a giant fish/shark was going to swim out of the darkness and eat me. Very primal feeling. It actually motivated me to start working with a teacher. The teacher said, really casually, "Cool! Whatever the shark eats isn't you." Between the playful tone of his voice and his not being worried about the situation, I approached my next sits like "go ahead, eat away whatever isn't me. I want to see what is left."

Fear mostly reveals your desire to protect yourself. If you can see that as an impersonal mechanism that is playing itself out in the "laboratory" of meditation, then you can be more of an observer and less personally scared. It will still be scary but also with a flavor of excitement and exploration.