Basic Convolution Reverb Question I'm surveying for feasibility techniques for a potential future project that's rather out of my past areas of expertise, and I have a fairly basic question. Of course if it looks feasible I will have to learn a whole lot more on the topic.

What is the equation to compute the number of FFTs/IFFTs per sample required, given the number of samples in the FFT n, the number of samples in the FIR m, and the number of samples of audio data this is applied to at a time x?
Depends on your algorithm. Assuming overlap add/save and a filter length of N we would get: 


Zero pad to 2*N and forward FFT: 4*N*log2(2*N) multiplies
Spectrum multiply: 8*N multiplies (4*N if input and filter are real)
Inverse FFT: same as 1
Overhead for padding, overlap handling, house keeping etc.


There are between 1 and 1.5 times the number of adds. There are also various ways to optimize this. 

In general, convolution is not always the best way to create reverb. It is computationally expensive, it adds a lot of latency, and it is very difficult to update or modify in real time.