I depends on what your goals are, and as mentioned, how much time you have to train. Unless your a poor swimmer (relative to bike & run), a good ratio is around 50%,30%, 20% (Bike, Run, Swim).
Lately on my higher volume weeks, I'm around 52%, 31%, 17%.
You also have to consider 1) What you can recover from in terms of swim & bike volume. These both can take years to develop the skills and specific fitness. 2) how bad you are at swimming or cycling.
I personally think it's best to run 5-6 days a week and keep the longest run <1/3 of the weekly volume with exception to a race specific build for long course.
Triathlon should be an eye opener. If you get serious, you'll realize that run only training is easy in that you realistically can only put in maybe 8-10 hours per week at most. While triathlon, you can work up to over 20 hours per week that might still include 6+ hours of running. But it takes time and hard work to get there.
TrainingBible Coaching
http://www.trainingbible.com