If my math serves me right this early hour, the timeline for Oxymetazoline should look like this:
Time: 6h 12h 18h 24h 30h 36h 42h 48h
Activity: 1/2 1/4 1/8 1/16 1/32 1/64 1/128 1/256
So after 12h the effects are gone and after 24h / 48h you should have an activity of 6,25% / 0,39%.
I deem 2 days for me to be safe, however I never received medical training.