It's pretty clear. "Out of memory" means that you've reached the maximum size of sound buffer that you can build.
Correct me if I'm wrong, but 90 minutes of uncompressed stereo audio at 44100 Hz takes almost 1 GB, which is huge
SFML may not be the best library for saving big audio files. You'd better find something that can directly encode and write chunks of recorded audio to the target file.