I know it can sound strange and I don't know how it can be done. I don't know how to get the pulse duration only with an interrupt in the FALLLING. Even I don't know if it is possible.
I really don't see how you can tell how long a pin has been HIGH knowing only when it went LOW.
If you know when it went HIGH and when it went LOW, then it is trivial to determine how long it was high.
If you know that the signal is periodic, that is that it will be HIGH for just as long as it is LOW, then you can time the interval between two FALLING edges and divide by two.
If you know something more about the signal than you are telling us, now is the time to do so.