I ourchased a whole load of cheapies from ebay came with 40 OPKs and 10 HPT.
I have only tested after CD10 as my cycles generally in yhe longer side and have stopped using the OPKS 3 days after the + mine got from dark to barely there within 24hrs.
Everyone's LH levels will be different so some ladies well have a steady increase and decrease but mine got from pale to intense within 24hrs and then back to pale! So after another 2 days I just stop.
The fertility expert I spoke to said a lot of women will continue to have surges on and off post ovulation int he 2nd half of the cycle that is normal and usually a durge a few days before AF shows up.
I learned that last on CD 26 I got a LH surge that showed up before the control and then 3 days later my period started.