I would say roughly from 16th oct. Its correct it depends on your body but assuming its a 28 day regular cycle
It just depends on your body. Some produce more HCG and sooner than others. Rule of thumb is day after ur missed period. I tested the day after my missed period got a bfn. Retested 4days later and got BFP. A lot of ppl are POAS addicts (pee on a stick) and test every day. Hope this helps :-) good luck to you Hun.