I have a laptop harddrive that is about 5 years old. I backup to a USB drive every other day. It is inevitable that this drive will fail at some point. When

I have a laptop harddrive that is about 5 years old. I backup to a USB drive every other day. It is inevitable that this drive will fail at some point. When is the optimal time to buy a replacement drive? I have always assumed that I should just get one when my current one fails, but this could come at an bad time (when I'm travelling etc.) Is it ever better to just buy a new HD when I have a chance to mess with doing reinstall?