There are many PR schemes under which you can apply, and all of them have different requirements. For the Foreign Students scheme, you must be living in Singapore for at least two years to become eligible for PR application. For other schemes, there is no such minimum duration of stay required. However, you must provide salary slips of your last six months from your Singapore employer for the PTS scheme. This means you have to wait for at least six months before you have all the required documents to apply.
In general, we recommend waiting for 1-2 years before submitting your PR application.