Generally, for African Dwarf frogs, heights over 14" are said to be done cautiously.
The height may be fine for a few months...but things happen and for one reason or another the frog can get ill or stressed. It may become a struggle to breathe, having to climb over 14X your own body length to breathe every few minutes. This takes quite a bit of energy. An ill frog may die simply from being too exhausted to reach the surface to breathe.
If you have a filtration current, this is an added struggle to reach the surface. They have to fight the current every time they go to take a breath.