How often do small dogs need to be walked a week?

My younger sister has a Maltese puppy, she is only 8-9months old and very small for her age. She maintains a lot of energy and seems significantly happier when she's had a bit of exercise. My aunty in England who recently moved to New Zealand has a small Terrior of some kind and walks her twice a day. I think daily is the best, but its really just as much as you can. I would say 3 times a week at the very least.

Also, Roxy (our Maltese) had trouble walking on a lead at first, it turns out that not looking back at her was the trick if that's any help.