No cycle this month

Ok so my cycle is always irregular so I didn't think anything about it when it was a few days late but this time it never came my start day is the 12th or the 18th of each month or I might get it on the 20th but this time nothing at all well I experienced a little spotting but it was only one drop of blood and it only lasted one day I wiped and whiped trying to see more but there was nothing I'm nervous and happy at the same time so my question is how long should I wait to test