Ann: Unaudited Results for the 9 Months Ended 31 March 2016-TFC.AX, page-46

Roy2U, I would guess the 15 year harvest timeframe was somewhat arbitrary when they began to offer MIS investments all those years ago. Balancing investor's potential need for a return on investment with hoped-for (at the time!) yields of oil.

If I could take you waaay back to 2011 when TFC announced trial harvest results:
http://www.asx.com.au/asxpdf/20110603/pdf/41z171cqkv41s2.pdf
The report indicated a high correlation of heartwood (and thus oil) yield to tree size, and so from that the longer trees are left to grow the greater the yield. Which seems logical....double tree height, heartwood volume should increase by 8x.

So it would be better for TFC to leave its own plantations in the ground as long as possible, but whether customer (and shareholder) needs will met by such an approach will be seen.