You can apply after 7 days have passed since your positive Covid-19 test, provided you have been free of symptoms for 48 hours. If you are still unwell after 7 days then it's unlikely you have recovered yet. Your symptoms will be assessed during your application.
The day of your test counts as Day 0. For example, if you tested positive on the 1st of the month, you could apply for your recovery certificate on the 8th.