Length of your animation should be set within animation node, not the player. Try something like this.
var length = 3.0
var your_animation = Animation.new()
your_animation.track_insert_key ( 0, 0, Vector2(0, 0))
your_animation.track_insert_key ( 0, length, Vector2(128, 0))
Basicly you create custom length variable and assign it to time argument when inserting new key to track. With continous updateMode it should work with any amout of frames, if Im not mistaken.
After animation is set, you add it to your player by add_animation() function.
Have a research here: https://docs.godotengine.org/en/3.0/classes/class_animation.html?highlight=add_track