The average young, fertile couple has about a 25% chance of getting pregnant each month. Things that hinder your chances include the woman being too thin or too heavy, not getting enough sleep, being stressed/sick, not having sex during ovulation (mid-cycle), and obviously, infertility. The general rule of thumb for a couple in your age range is to try for a year before seeing a fertility specialist.
I've read many of your posts and you seem to be fairly conservative in your values, so I'm curious as to why it's you and your girlfriend trying to get pregnant as opposed to you and your wife. Are you planning to get married? Not judging, just really curious about the value system you've got going on.
Anyway, good luck.