Basic Convolution Reverb Question
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.