When can my wife test?

So I am going to try to get her lingo down. Lol. So as of today I think she said she is 18 days past ovulation. She should have started her period four days ago, but didn't so we are really hoping that this will finally be our time. She tested the day after she missed her period and also the next day and both were negative, unfortunately. :-( The first test was in the afternoon and the second was in the morning. She said something about implantation?? That if it occured later then her hormones may not be built up enough.....so I am asking when is a good time for her to test? Like when would it for sure show up she is pregnant? We don't really want to get a blood test just yet, because we thought she was back in September and she wasn't and come to find out she wasn't releasing an egg, but now she is. So can someone also explain how this hormone builds up day by day? Like with numbers. And when it would show up on a test. Thanks so much to all you ladies. You help her a lot! And don't laugh at me for asking...sorry if I got something off.