ADS-B transponders are also not yet required on all aircraft outside of Europe so many planes don't have them and won't show up on this site.
Some airline pilots will only activate certain modes of their ADS-B transponders in flight after take-off, particular those used to broadcast velocity and position, so it's possible that's what happened in your case.
For all other cases the upload from the ADS-B receiver to the Flightradar24 site will be as real-time as is possible with standard internet infrastructure.
Lots of stuff is happening in other domains too..
In Switzerland, the federal aviation authorities tried to impose a delay as well. After a wave of protest pointing out the absurdity of such security through obscurity – you can see aircraft in the sky after all! -, a delay is no longer necessary and you can check in real time which aircraft is flying over your head at the moment (if equipped with ADS-B).
Mind-blowing accuracy actually. Sky is clear out so I just looked out my window, saw two planes going in opposite directions.
Looked at the site, and there they were. Really cool!