How likely is pregnancy with "pull out method"?

i recently stopped taking my BC pills (progesterone only) because my cycle became very erratic. my doctor suggested that i stop taking them for a few months to "restart" my cycle. i decided not to go back on them because my husband and i are planning getting pregnant in the next year. although right now, we are not actively trying, we are not using any contraception and he just pulls out. i know pregnancy can occur using this "method" but i am wondering how common it is?