Hi,
When breastfeeding, there's no need to actively cut feeds out as babies will naturally adjust their milk intake as they take more food. At this age, as you say, it's the milk that's more important to nutrition, so if you were to actively cut breastmilk from the diet, it should be replaced with formula. As baby approaches 12 months, this will change.
As for when.... well it depends on the baby. 7 months is still quite young, you're likely to see a significant change in the next 2-3 months.

Babies do reduce their milk intake naturally, though they don't necessarily reduce the number of feeds (just take less per feed). However, after 12 months you can cut feeds without having to introduce formula if your baby is eating food.

ETA - my son was 'eating' by 8 months, my daughter about 10 months.