That number can actually vary. Your metabolism and other factors can affect how slowly or quickly LH works out of your system or even builds up which can make it look like a long surge. I typically O'd the day of my last positive OPK, but one cycle I O'd the day of my 3rd positive (I had 4 that cycle) and others I O'd the day after. To be safe I would start your 2WW the day after your last positive OPK. Charting would definitely help you know what's going on better though. You may see a noticeable dip on O day in addition to the OPK and a spike would give you certainty of when your 2WW started.
Good luck!