How Common Are Erratic Menstrual Cycles When Trying to Conceive?

I'm not sure how common it is because it isn't really a big thing I've looked into.... But how many other people deal with ridiculously erratic cycles? My cycle can last anywhere between 12 and 40+ days. I'm 23 and my doctor doesn't make it out to be a big deal. For me it's really aggravating because while DH and I are trying to conceive, this causes a drastic difference when trying to predict ovulation. Any tips to try and figure out the best time to try?