A lot depends on the anticipated time from peak to peak, and the format you want it in. If the time domain is fairly predictable, and you want a digital output, then I would suggest using a fixed clock, an op amp, a pair of flipflops, a couple of 8 bit counters, and a schmitt triggered nands. With this logic you can detect the first "voltage" peak and start countingby toggling a F/F, then with the second peak have the second F/F "latch" the counter's outputs and reset the circuit.
A partial components list might include:
74x74
74x132
74x579
x741
an X-Tal or other clock source
The use of an MCU could decrease component cost and count while increasing accuracy and flexibility, but would require some coding ability on your part.