First month using OPK

Lindsay • 👦🏼 1.31.13 👼🏻 12.26.13 👦🏼 👦🏼 4.27.15 ❤️Currently expecting baby #4! 🤰🏼

When should I start using opk? I haven’t ever used them before so give me all the input you have! They are so pricy, so I’m afraid of starting them too early. Just want to get my timing right and not spend tons of money every month on these tests.