Not an easy question to answer. Take Sitcom as an example. You may be offered an option fee of a couple of hundred pounds while a producer hawkes your idea round the broadcasters. If they can get one interested, they'll commission a pilot script, for which the fee might be £7k. This will likely be paid in stages and the final 25% could depend on the show being commissioned off the back of the script. If not, you won't get all the fee.
If the show is commissioned, the £7k will be about the fee for each of 6 episodes. Then if it's repeated you get a certain percentage of the fee again, depending if it's a 1st or subsequent repeat, prime time, and which channel etc. If it gets released on DVD or the show is sold abroad then there should be small percentages in the contract for you as well in those instances.
If the producer/broadcaster pulls in more writers your cut reduces.
All of this assumes you get a good contract in the first place.