Well, even if the title is writen like a question, i am actually here to answer it.
I made a new character and measured the time needed to get to max level (with main quest only) wich is curently as of "march 2021" lvl 60 and HM12.
Yes, with the main quest line you cannot go over HM12, i did not use any charms to get higher HM level altough i did a few sideq, at the end those 2 where you get
"legendary gem hammer" and ofc all hongmoon purple quests for more stamina (i bought the items necesarry from the marketplace).
I did include loading times since its part of it (got an Samsung SSD 970 PRO).
But now to the times, since i cannot (family, work ect..) play all day, i had to cut it in levels, every day 1-3h of gaming.
lvl1-9= 13min
lvl9-20= 85min
lvl20-28= 43min
lvl28-36= 55min
lvl36-41= 76min
lvl41-43= 50min
lvl43-46= 61min
lvl46-48= 97min
lvl48-51= 95min
lvl51-54= 45min
lvl54-58= 102min
lvl58-59= 18min
lvl59-60= 28min
lvl1-60 = 768min / = 12h 48min
HM1-3= 30min
HM3-4= 11min
HM4-7=63min
HM7-9= 68min
HM9-12=123min
HM12----last m. quest= 43min
HM1-12(last m. quest) = 338min / = 5h 38min TOTAL: 1106min / = 18h 26min
Btw. i played the archer class, some classes could be faster or slower, depends...
...also i did not read any of the dialogues ofc, here and there put new eq on wich i did when some NPC were talking to not lose time.
Also having premium helps at the beginning, since you windwalk alot and it happened a few times that i had to wait 1-2 minutes for the cd.
So yeah, im sure it can be done faster but overall thats it, it will take that much to finish the main quest line +/- 1-3h... depends on loading times, class ect...