For me, onset of pain was 24 to 48 hours later. Delayed feedback is not very effective at driving behavior modification. But that's another story.

I recall reading somewhere along the back pain journey that every spine is different, so personal testimony is of limited experience. Cadaver studies of people who did and did not experience back pain found little correlation between reported pain and common physical spinal conditions.

So take everything with a grain of salt.

Lowering my saddle near the limits of conventional wisdom regarding knee angle helped my spine immensely. Occasional mid ride stretching helps my spine. Indeed I've found a too upright position is bad, but also too low. My saddle to bar drop is zero, and I can ride literally hundreds of miles moving between flats and drops. That's my spine.