Ugh. With the twins, it returned for me at 7 months PP. They were exclusively nursing, but had started a tiny, tiny bit of solids. They were not co-sleeping, but woke up all.the.time to nurse. They did not use pacifiers and comfort nursed. I was bitter!
For my singleton, it returned at 3 months PP, with an episode of bleeding at 6 weeks PP that may well have been a period, too. He co-sleeps, had not started solids at the time and only very occasionally goes even 4 hours without nursing. I think one night he slept 8 hours, but other than that, wakes every 2. Nurses all night. Never even offered a pacifier.
My sister had her cycles return at 4 months PP with both kids. Exclusively nursing, no pacifiers with her oldest, but no co-sleeping for either. Had not nightweaned at 4 months PP, though.