Leaders are used in either two ways. 1. Is the be abrasion resistant if fishing toothy critters or fishing around structure. 2. To minimize line visibility normally with fluorocarbon leader.

Under the 1st scenario it depends on what you are fishing for and where. If your using 8-10lb line then a 20-30lb leader usually does the trick. Never go lighter than line rating spooled on reel. Latter scenario you can use spooled line rating or a little over. This is where line resistance isn't in play but needin to get easily spooked fish to hit and not see your line.