The regulations will determine what happens although some airlines may have stricter currency rules than the legal minimums. My airline has a well set out process and what, if any, training is needed and it is based on the time off between flights. Most pilots will need either some flights with a trainer or time in a simulator. Recently qualified pilots may require more training than more experienced ones. For my outfit the training department are working out the requirement for each individual pilot. A rather difficult task I imagine! Hopefully it will come back quickly for most of us and fingers crossed we get to fly again soon.