A full binary tree is a binary tree in which every node has either 0 or 2 children. If a full binary tree contains both left and right sides as max heap, then the time complexity of making it a proper max heap will be O(n) where n is the number of nodes in the tree.
This is because in a full binary tree, each node has two children, and to convert it into a proper max heap, we need to compare the value of each node with its children, swap it if necessary and repeat the process for all the nodes in the tree. Since each node has two children, and the tree is full, we have to repeat this process n times, and hence the time complexity is O(n).