How does the time taken for backpropagation change with the increase in neurons?
The general idea is that time complexity is related to the number of operations needed to perform all the steps of backpropagation. You can find an example workthrough here:
https://medium.com/swlh/computational-complexity-of-neural-networks-38c01e7e566a
For a 4 layer NN it seems the complexity is O(n⁵), where n is the number of nodes of each layer (also n is the same for every layer in that example).