Actually, it lasted on and off until she was about 2. It wasn't all the time, she would go for a few weeks and then have a week or two like you describe. Although if you're at all concerned, your HV would be able to put your mind at rest.
Mine said that it was something called toddler diarrhoea, it's not a tummy upset or a bug, it's just something that happens to a lot of toddlers.