I quite sure that it should be $O(n)$, but I didn't found any information about it and I'm not sure how to prove it. Maybe in 2-3 tree the max number of node (include the leaves) is $2n$ and in each node you pass maximum $4$ time. The upper limit is $O(n)$. So in B+ tree it's limit by $2*B$ times. Is it right?