Period a week late, two BFN down, one to go tomorrow.

Christine
Hi everyone, 
So my cycle is now a week late. I've been down with the flu all week with no real other symptoms of either AF or BFP. Going to take another test tomorrow but if BFN, how long do I wait before getting any sort of worried? My cycles can be irregular so I don't want to get my hopes up. Don't really know what my question is other than how do I not go crazy?  Haha.