Usually, a company needs about 3-6 months to find the right replacement.

Hence, the company should send you an official letter detailing the job scope, expectations, deliverables, replacement duration and additional compensation for you to take over the resignee's work.

You can decide if you want to accept it or not.

This is so that if you fail to deliver what the resignee is supposed to do, your own job KPI and performance appraisal will not be affected by the additional workload.