I bought a big pack of OPKs three cycles ago. The instructions said to use them within 30 days of opening. I used them for the first two cycles and got results that showed ovulation. The second cycle was slightly over 30 days from opening. I'm now using them for a third cycle and wondering if they are accurate or if I should buy new ones. From what I can tell, they appear to still work, but I haven't had my LH surge yet this cycle. Thanks!