You will be emailed 1-3 weeks ahead of time asking you to cover shifts. Differrent districts may vary. Sometimes you will get called the same day in cases of call offs/emergency. Typically the drive time was less than an hour but again this will vary by your location (urban or rural).
If they call/email asking you to cover at a store 2 hours away be sure to ask about reimbursement for mileage and hotel. There are some cvs stores in the middle of nowhere that are sometimes really busy so when their pharmacist goes on vacation corporate brings in floaters from a big city.