How late is late?!

Laura • Straight edge, vegetarian, feminist, lover of animals.
I was supposed to get my period yesterday but nothing from AF other than a little cramping on the right side. 
​I took a FRER the day before AF was supposed to show up and it was a BFN, then I also took one this morning FMU, day after AF was supposed to arrive, but again BFN.
​I have been reading a lot online and they say " wait till your period is late" 
​My question is.. How late is late? When should I restest?