Person-to-person basis 'yan dude. 'Yung iba pwedeng 28 days, pwedeng 1 week lang, pwedeng 40 days etc. Kapag regular, meaning, dapat consistent 'yung menses niya (+/- 7 or 8 (ata?). Dapat 'yung longest and shortest (ever) na cycle niya eh about less than 1 week lang ang pagitan. Example, eto ay menses ng babae sa 3 buwan (28days, 26days, 30 days)- Considered siya as regular. Ang example ng irregular eh 15days, 30 days, 12 days and so on.