A plant takes 18 days to grow all 8 seeds (and resources) with the seeds (& resources) growing the last 8 days.
If your after orange petals, cross pollinate pairs of orange bulrush plants the day they show as plants (stage 7), and the seeds you get from them, while labeled orange bulrush, will show bright orange bulrush once in a pot and grow bright orange plants with both seeds & orange petals.