Thank you for your responses. After contacting home office, it turns out that I will have to wait till 28 days prior to the current Leave to Remain.

Hi all. I have been in the UK for over 10 years legally. Currently I am on a 5 year Refugee Leave to Remain. I was wondering if I can apply for "indefinite leave to remain" now, or have to wait till 28 days prior to the end of the current leave to remain. Many thanks