Celeriac chips are best served straight from the oven. However, they can be kept for 4-5 days in an airtight container in the fridge. For best results, reheat in the oven. You can also freeze them for up to 3 months. However, they will be softer once defrosted and reheated.
If you want to prepare the chips without cooking them, then you can do so up to 24 hours in advance. Keep the sliced celeriac covered in a bowl of water in the fridge to prevent the skin from oxidising and turning brown. However, unprepared celeriac will keep for several weeks in the fridge, so you don't need to feel the need to use it quickly if you bought too much!