Here is a typical layout of one of my practise sessions - generally about an hour at a time, although sometimes slightly more:
5 minutes or so throwing at the bull (or the general area) to get the arm warmed up
10 minutes or so doing round the clock on doubles or trebles - I hit them twice before moving on (not necessarily in the same throw though)
15-20 minutes of finishing practise - either the 121 game or from 60 to 100 in 6 or less darts
15-20 minutes playing against no1 - generally 2 or 3 sets of best of 5
then I finish off with 10-15 minutes or so throwing purely at either the T20 or T19 mainly to warm down
I vary the round the clock and finish practise with a couple of other games such as Bovril's 100 darts at the 20 (I may go for the other high numbers of 17, 18 or 19 to practise the cover shots. I also occasionally just pick a random number and try to finish that as many times as I can in 3 or less darts for around 15 minutes.
As far as the round the clock part is concerned, I'm going to change that to if I don't hit the next number at least once within 6 darts then I have to start again.
I basically try and vary the middle parts as much as I can so that I don't get too bored just doing the same thing over and over again.
All I've got to learn now is how to take my practise game into a proper league match lol