As often as they want. Offer solids first then the breast. Here's an excerpt from kellymom.com:
Some toddlers are eating very few solids, or even no solids, at 12 months. This is not unusual and really depends on your child - there is quite a big variation. We like to see breastmilk making up the majority (around 75%) of baby's diet at 12 months. Some babies will be taking more solids by 12 months, but others will still be exclusively or almost-exclusively breastfed at this point. It is normal for baby to keep breastmilk as the primary part of his diet up until 18 months or even longer. An example of a nice gradual increase in solids would be 25% solids at 12 months, 50% solids at 18 months, and 80% solids at 24 months.
You can read the whole article here
at 9:43 PM on Jul. 24, 2010