I don't have them on a board anymore. And to answer your question, I think as long as you need it. This tree has a very wide base already and the roots are pretty much all around. I may need more grafts in the future is all I think it needs as far as roots go. I had a conversation with another member, and I think shohin will generally benefit having spent less time on the board. Larger bonsai could use much more time and likely grafts too!