OPK/LH surge help??

Amy
Hi,  :)
​I'm in month 3 of really charting and testing after 16 months TTC, my question is:
​How long is the LH surge detectable? I did an OPK last night around 6:30pm and the test line was as dark as the control line at the bottom but quite faint at the top, so I assumed this was a negative but getting there,I'd had cramping through the day which has happened recently with O. didn't get a chance to test again later but did another today a little before 6pm and it was an obvious negative. Could I have missed the surge in the 24 hour gap? 
​Obviously I'll look out for the temp spike but  is it worth getting my hopes up? Having PCOS means that I may not O every cycle so trying to get used to my bodies signs to finally get that BFP which seems so far away!
​Thanks x