There are some basic limitations regarding the length that a furled Tenkara line can be produced. One limitation is just having the necessary room to lay up the line and then furl it. The other limitation is that at some point, there is too much resistance and it becomes difficult to achieve an even and consistent twist throughout the entire length of the Tenkara line.
One option is to furl a multi-piece line which can be connected via loop to loop connection. This system actually provides some advantages regarding having the flexibility to easy rig up a longer or shorter line.
Here is a set up I have been using for warm water, still water fishing conditions.
Top left is a 10.5' level furled line that basically serves as a Tenkara line extension. Bottom right is a 10.5' furled weight forward tapered line, top right is a 10'-12' foot tapered single strand mono leader. The furled level extension and the furled weight forward tapered line are connected with a loop to loop connection
This gives me a total line length of about 33', couple with the reach supplied by the rod, I can fish out to approximately 38'.
Personally speaking, when I first started to fish this set up I was able to reach out over 35', but in order to do so, I needed to really concentrate on my Tenkara casting technique. As I fine tuned the leader/tippet, as well as limit the fly size, casting has become much more routine and predictable. As with most things, it just may be a matter of practice makes perfect (or at least close to it).
Of course, I still find it a much more pleasurable/enjoyable experience fishing a more normal sub 20' Tenkara set up. When fishing a normal set up, casting happens almost sub consciously. I basically look, see a spot that I want to fish, and bam, the fly shows up. My total concentration is on the water. When casting the "magnum" set up, I really need to focus more on the physical act of casting.
I use the "magnum" line when fishing still water ponds/lakes, primarily from shore. Because of the way I fish, it really offers no advantages when I fish moving waters. When fishing rivers and streams, a conventional length line is what I use.