This depends on whether you are married to a British citizen.
If you are married to a British citizen, you can apply for naturalisation as soon as you have obtained Indefinite Leave to Remain. However, you will need to satisfy the requirements for British citizenship below.
If you are not married to a British citizen, you must not have been subject to immigration time restrictions for at least 12 months prior to an application for naturalisation. This is likely to mean that you will only be able to apply for British citizenship 12 months after your granting of Indefinite Leave to Remain.