Basically, ink lasts for quite a while if you treat it right, e.g. not leaving the cap off or loose, not spilling much and not using a brush to slather it onto the page. There are people gleefullly using 70 year old ink.
For someone in primary school I think that one bottle of ink, probably a washable blue for that age range, will usually last at least a year.
I've been out of primary school for a little while, so I don't recall exactly what my ink usage rate was back then, but I'm sure that I used very little. That has probably not changed for the current primary schoolers.
He said primarily for school. Not that he is in primary school haha. I was using crayons in primary school, aaahhh good times.