Advice on when to start opk testing pls

Nikki • Pregnant 💚
I have irregular cycles (cycle length for the past five months have been 30, 40, 34, 34, 30 days) so I'm hoping for some advice on when I should start using my new Clearblue digital fertility monitor. Af started on 9 Jan and has gone now but I'm conscious that the test sticks are quite pricey! Anyone been in this position and have some advice on when to start testing?