A 10-seconds digital reverb with very few memory (64KB) I'm currently thinking about the design of a low-cost digital reverb guitar pedal, using a Teensy or Arduino.

My goal is to achieve big 10-seconds long reverb.

If I implement reverb with a mix of many delay lines, this will require to store in RAM at least 10*44100*2bytes ~ 0.8 MB of audio data (44.1Khz, 16bits, mono), so it seems to be impossible to achieve with a 64KB RAM board (am I wrong?).

My question: Is there a more clever way (using FFT ? overlap-add stuff, or anything else?) to implement 10 seconds reverb with very little RAM (64KB)?
Take a look at this https://ccrma.stanford.edu/~jos/pasp/Freeverb.html or https://github.com/highfidelity/gverb. Gverb is clearly the better sounding one but harder to implement.

Not sure if you can squeeze it into 64kB but it's worth a shot. The length of the delay lines is scalable to some extent. It roughly corresponds to room size with small delays being small room. 10s in a small room sounds more like bathroom and less like a concert hall or a cathedral. For really high reverb times I would recommend a floating point implementation to avoid instability and noise in the feedback loops.