I suspect, though it's just a guess based on the road/tri clubs I train with, that for most dedicated triathletes it is close to 100% on tri. There may be a few triathletes who also ride road bikes (or MTB or 'cross or 'bents or track or fixies or unis for that matter), but few; and even then, most of their mileage - if tri is their focus - comes on a tri bike.

As trisooner said, most triatheletes train on the bike they will race on. This is for several reasons: the first, since you will be ridding the tri bike for over 100 miles it is best to train that distance on the bike you will use, so that you are comfortable and accustom to all the nuances of that bike during that distance. Secondly, your transition, during bricks, to running will differ due to the geometry of a tribike vs.road bike. Therefore, for IM it is best to train as you will race.
I ride both road and Tri, however, I only use my road when ridding in large groups that do a lot of drafting, usually once a week and I use the tri bike for race specific riding at least 2- 3 times a week and with brick workouts.

Sold the tri bike two years ago. 60% Road bike, 40% MTB (ridden on the road.)

I ride my

Tri bike- on EVERY long ride (65-125 mi) Almost 80% of my total mileage comes from this bikes computer.

Fixie - every Thurs nite for our bike clubs beer run (35mi round trip and the occasional alley kat) and for 60% of "A group" fast rides. (Really forces you to man up on the fast sections)

Road bike - when riding with roadie groups that dont allow aero bars. (1-2 rides a week)

MTB- 1-2 times a week for a few hours on some really hard technical sandy trails.

Planning to add a cyclocross bike this winter to possibly replace my aging road bike.

+1