For a stern line, I like at least 200 feet, and have used up to 300 feet. I presume that you mean a line for taking ashore to a tree or rocks. I also carry about 15 feet of 3/16" chain, with a couple of D shackles for the stern line, as well as 25 feet of heavy poly propylene line with eye places in both ends. The poly propylene is easier on trees than the chain.
If there is a high tide, or the beach is dicey--it is nice to be able to slip the line from the dinghy without going back ashore.
I have used both light nylon line, and floating polypropylene line for the stern line.
Others may have different amounts. I have carried these lines on hose or plywood rope reels, as well as in standard coils.