It varies incredibly from woman to woman.....the "normal" cycle is supposedly 28 days, but can be anything up to 45 days. Lots of things can affect cycle length, inclusing illness, some medication, termination and mc.
Mine have alwaus been a bit randon, especially as a teenager (I could go for 6 months with no bleed and then bleed for 3 weeks!)
At the moment they seem to be around the 29-31 days mark.
HTH