How long after suspected IB would you test?

I have extremely irregular cycles so my next expected AF could be tomorrow or it could be a month from now, so timing the test around a missed period would be almost impossible. I had some pinkish brown spotting, that could possibly be IB. If it was, how long would I need to wait for HCG levels to rise? I don't want to waste my time and money testing too early, but I can't exactly wait for a missed period not knowing when that would be.