The length of spawning a 131,072 tile will take about 18 hours and a few extra minutes before the 4 becomes the last number after 36 hours, if 262,144 tile is ever achieved on a 4x4 grid, the length of the time will about 3 days an extra hour to despair and the last spawning number would be an 8 but nothing else could be spawned until you know it. The 5x5 tile is recommended to prevent running out of space because the classic game is a 4x4 grid of the original design game but
for advanced levels 5x5 grid or will achieve it without running out of tile spaces.

Excuse me, the highest tile that you will across with is 131,072 tile on a 4x4 grid and I would doubt any tiles to remain higher than that because if you run out of space the game will end. And a maximum score isn't likely to exceed 3,932,160 points, so I recommend you to use a 5x5 grid in order to see a 262,144 because this cannot be spawned on a 4x4 grid without hacking or using any cheats, but the lowest should start with a 2 or 4, but it would be more likely even impossible to expect 262,144 tile to be seen on 4x4 without getting game over. But if you spawn it on a 4x4 grid, then you be the luckiest candidate for that responsibility but will not exceed a possible score of 8,388,608 points, but more likely will run out of space around 7,864,320 points little bit short of 8 million because 8 will be the last spawning number even a 4 wouldn't be spawned without getting game over.