i think u meant pre-employment screening as in the G50 screening?
or u including both the G50 and health screening

G50 --> depend on nature of your potential organisation, could take minimum 4-6 weeks?
no one has definite answer to the exact timeline, it's all ball park estimates 'cause the G50 screening remains shrouded in mystery tbh on what they screen

health screening --> assuming by then u 1/2 foot into the organisation, normal procedure to do health screening at the pre-assigned clinic to make sure u body parts bor daiji and can carry duties lor..so far quite rare to hear (though not say no have) retracting of job offers due to failed medical.