Rod,
I think it depends on what race you want to peak for and what races you want to basically use as training. I've done a local medium sprint race (Swim .5mi; Bike 16mi; Run 5mi) on a Saturday morning, then an unofficial (by myself) swim of .5mi , run 6mi, bike 18mi, in the afternoon. Then did a 70.3 on Sunday morning, for my own personal weekend Triple-T. I've also done sprints on the last wkend of June, a midweek July 4th race, and then another sprint on the first wkend of July, to make 3 races in an 8-day period, no problem. These true sprints take about 1hr 20min to 1hr 30min, not really a very long intense workout when you think of it.
Have fun and toy around with the sched. There are so many fun races these days, if you only do 3/yr you'll never get to them all. Much like golf courses!!
tim