Typically 20minutes per 500g plus 20min. So for a 1.5kg ham 4x 20mins, that is about 1 and a half hours.
Note it takes quite a while for the ham to reach the same temperature as the water so you need to calculate the time from when it is boiling quite hard, not just a few bubbles.
If you are finding it too salty pour off half the water after the first 45 mins and refill with boiling hot water.
I have found if I overlook it goes a bit crumbly so a touch of trial and error needed but also most butchers use brine injection to create their hams quicker, that may mean a different texture but at the expense of flavour.