What do you want to take into account into your estimation? Also the harvesting of the fibres, treatment of the fibres, sterilisation and other preparation? The growing time is between 5 to 25 days, it can go to 40 days. Choosing the proper growth time depends on a lot of factors, not only the environmental conditions, but also the desired properties of the application (thermal conductivity, water repellence, compressive strength, flexural strength).
With what type of fibre do you want to work? Calculate the desired density, so the fibre mixed with water (the amount is about 300%H2O of the dry weight of the fibre, depending on the fibre-type and size), and you'll know how much you need. It depends how much you want to compress the panels, depending on the application. Is it for acoustic isolation or aesthetics?