When baking several potatoes at the same time... try to make sure that they are all of a similar size and shape, that way they will all be ready together.
Unless you buy pre washed potatoes you will need to scrub your baking potatoes thoroughly to get rid of any dirt then dry them well, leave them aside to dry as much as possible.
Now, prick the potato skins a few times with a fork and rub few drops of olive oil all over the skin of each potato.
After that, rub in some crushed sea salt to make the potato skins more crisp when cooked.
Potatoes which are between 350g and 400g in weight will take about an hour to cook in the oven
Put the potatoes straight on to the middle shelf of the oven, pre-heated to about 200°C and let them bake for 1 ½ to 2 hours depending on the size of the potatoes, or until the skins are really very crisp.
(Source: Baked Potatoes)