Whenever they need them. Mine are little and they grow quickly so I buy new stuff whenever I notice them growing out of the old (every couple of months). Things like party dresses I tend to give my daughter for her birthday or Christmas so she appreciates them more.
Depends on their age, I think. Kids under 10 years old tend to grow pretty fast, so they won't fit their clothes after a while. Other than that, you can buy clothes for them whenever they need new clothes, depending on your budget.