How do you know your "Average Cycle Length" with irregular period

Ruthie • Mother of a handsome lil boy, he is 8 I`m trying to have another child
My period is VERY irregular. I can go 2-3 months without a period. So how am I suppose know how many days to put when it asks for "Average Cycle Length" ? Ttc baby #2, been trying for 2 years.