Does anyone know of how the Ranma 1/2 timeline looks like,
especialy in regards to Ukyo and Ryouga?

Anou... If you're going to spell 'Ryouga' as that (with 'ou') you should
spell 'Ukyo' as 'Ukyou'...

Yrs old - event

-134 - Happousai
-84  - Cologne

0 - Ranma, Ryouga, Ukyo born

Most of the cast was born in year 0, too. This is also when Ranma got engaged
to a Tendou girl, when he was born.

6 - Happosai gets dumped in cave

No, this was before Ranma was travelling. Genma had hair at the time, and 
was still with Soun. At six, Genma has left Soun ages ago and was 
travelling around the place with Ranma. In the picture Soun is still 
young, too... Probably just after Ranma and Akane were born .. 2 or so?

6?? - Ranma chooses Okonomiyaki over Ukyo

Yep, Ukyou says it was ten years ago that that happened. Genma engaged Ranma
to Ukyou at 6, too.

??? - Ranma leaves for china, leaving behind an irate Ryouga?

Maybe 14 or 15... Though I think 15, because it looks like they were in high
school together.

16 - Ranma meets the Tendos, present day adventures and such....

That sounds right. They stay at 16 for nine years... you can tell by the
seasons changing. I guess they hit a stasis field in time... ^_^

Kun-chan...