: If you are looking for the length of the game overall, reaching 100% completion in BTD6 is a very "long feature," estimated to take roughly 591 hours of gameplay.
: Version 18.0 introduced new maps, but players frequently use the "long feature" of the Map Editor (introduced in later versions) to create custom paths. The longest possible path is achieved by dragging path nodes diagonally across the map to maximize travel distance; a single Red Bloon can take approximately 8 minutes to traverse such a path. Bloons TD 6 18.1.2747
For more specific details on version 18.x features, you can check the Official Ninja Kiwi Support or the Ninja Kiwi Twitter for historical patch logs. : If you are looking for the length