FWIW it seems to take about a year of consistent hard work in most cases. Obviously, the more consistent time invested the quicker it tends to go. That's no guarantee, of course
So true Roy but other factors will also dictate how easy it is to learn.
How long/easy it takes to learn depends on other skill sets learned and the depth of related knowledge or the player.
SN and Tab use the same skill set, they are both forms of cognitive reasoning based on Cognitive Pattern Recognition.
We associate and cross reference cognitive information so it becomes intuitive. We can learn very quick what the intention is using a cross reference of what we already know.
Chord sheets, Nashville Number System, crib notes etc all use this skill set, as human beings we cannot stop it, it is a natural part of learning and we cannot stop learning. We take in information in so many ways and it all links up to form our final reasoning or response to situations or tasks.
If a tab shows E string 5th fret then it is an A we play, the same as if someone says A, or a chord sheet says A, or in a numbers system in E by going to the IV we know it is A, or a Maj second of G is an A etc.
Regardless of how it comes to you, you will relate the infor to what you know. So in all the examples you understand it is an A that is to be played and you will play that A the same way you have always done regardless of where it is shown in a tab. You also know that the 5th fret of the E string is A, so the Information is supporting both ideas and coming to the same conclusion of what you play.
This is why some learn faster than others, they cross reference it better with what they already know so it becomes intuitive to learn. For others they have to start with no reference at all to base what they are learning. For example red means stop, green means go, once that information is known you will cross reference it into other situation to move or preceded or wait and stop. If you know red means danger you will cross reference that in situations and add that to stop, so you know to be careful or or use caution if you see these signals being used in other situations. So we see a green light on an appliance we always assume it is on, as green means go or OK, of it is red then it is off.
This reasoning is part of learning that's how we do it, so if we learn to read SN we will cross reference it with what we already know about music, and if we learn tab we will cross reference it with what we know.
If the common experience between the two is the fret board then we will associate the three together to make the final decision on the note to be played regardless of how it is notated.
This is why player can transpose in their heads what they see when reading different clefs, the cross reference the actual clef with the new clef by assigning the notes a new relative value and play them.
We all do it with key sigs when we read, we see the number of sharps or flats and adjust the fretting to suit. In many cases we do not count the number of sharps or flats, we associate their position and shape in relation to the last sharp or flat. Because we know the order they are written we assume that if the last sharp is an A we assume we are in Bmajor because we know that Bmajor has 5 sharps and the last one is A.
Learning to read is a multi skilled process, the more skills we have relating to it, then the better and quicker we learn it as the information supports or fills in the blanks to what we already have learned.