Can you keep milk in a plastic drinks bottle in the fridge or will it go off quicker?

You can keep milk in a plastic drinks bottle in the fridge, but it's best to use the original container if possible. Milk is sensitive to light and air, which can accelerate spoilage; the original opaque cartons are designed to protect it from these factors. If you transfer it to a clean, opaque plastic bottle, it should last as long as it would in its original packaging, provided it's kept sealed and refrigerated.