A multinational creates a new clothing brand to be sold from 1 January 2022 until 31 December 2034. Product development and marketing begin on 1 January 2020. The cost of bringing the products to market comprises of £9 million payable on 1 January 2020 and £12 million payable continuously during 2021. From 1 January 2022, when the textiles are in production, it is assumed that income will be received half-yearly in arrears at a rate of £5 million per annum.

Consider an effective rate of interest of 9% per annum. What is the discounted payback period, starting with the initial cash flow on 1 January 2020?