The time it takes for a fuse to melt is known as its "opening time." Opening times vary depending on the type of fuse and the amount of current that is flowing through it. For example, a fast-blow fuse will open in less than one second when exposed to a current that exceeds its rating. On the other hand, a slow-blow fuse may take several seconds to open under the same conditions. In general, fuses are designed to last many years without needing to be replaced. However, if a circuit experiences frequent power surges or other abnormal conditions, the fuse may need to be replaced more frequently.