There's a lot of reasons that your dog could be licking your feet all time.
It could be that the dog is just bored, loves the smell and taste of your feet or could be just trying to get your attention about something.
Another reason your dog could like licking your feet all the time could because when you sweat you put out salt through your body and the dog could be licking the salt from your feet.
I haven't had a dog lick my feet yet because I'm not near dogs when I have my shoes off but I've had dogs sit in my lap and lick the sweat off my arms and hands after being outside in the hot sun and sweating.
You could try to get the dog to stop the behavior if you don't like it otherwise it's nothing to worry about and the dog saliva can be washed off your feet and hands.