The amount of time you should boil your potatoes depends on their size, variety and how you want them.
Usually, old potatoes of medium size take about 25-30 min. to boil, with their peels.
Peeled old potatoes, of medium size - about 20-25 min.
Chopped old potatoes take about 10-15 min.
Fresh potatoes, with their peels, take about 10-15 min. to boil but it does also depend on their size.