To be honest this varies widely, especially between countries.
The speed of availability after something has aired is more likely down to contractual agreements between Apple and content owners than Apple - there may be contractual reasons why a show will air on a particular cable/satellite/terrestrial channel well before it is available in iTunes.
For example in UK say I had a Dr Who Season Pass then episodes are normally available a week after they've aired on the BBC. I think US users may get them even earlier.
I have purchased some seasons of Jake and the Neverland Pirates for the kids - episodes for Season 3 have appeared sporadically and slowly despite being available for ages in US.
If in US you will probabbly get the best deal in terms of speed of availability, just be aware that not all content you have on cable will necessarily be available in itunes (at least that applies in UK - I can't get a load of HBO content for example).