When Should Pregnancy Test Be Taken After Missing A Period?

It's been 51 days since the last day of my last period. I have long gaps between periods, so when should I be concerned enough to test for pregnancy? It seems everytime we think I'm pregnant, I start my period within a day of testing. I've had pms symptoms for over 2 weeks. My breasts are sore, I'm emotional, and have had a queezy stomach. I'm also getting over the flu. Any advice?