do you mean your cycle length?
most peoples cycles vary month to month as when we ovulate varies. what I didnt realize was that the ovulation date changes, but our LP (day after ovulation to day before period) stays much the same....mine is usually 14 days and once was 13 days. so if you know when you have ovulated (by ov tests or temping) and how long your LP is you should be able to guess your cycle length.
if you mean period length that is normal too, depends how thick your lining got in the cycle.