I am i thailand, I will go to lao begining ocober 2009 for a double entry TR visa.

then i come back to pattaya.

if i leave thailand after a month(end octobre), how long can I stay in europe, and still being able to come back for my second entry in thailand?

Vientiane issued two entry tourist visas have a validity for entry of 3 months from day of issue.

From the date of issue you will have 90 days to use the 2nd entry.

You might consider waiting to get your visa while you are in Europe. You could get a 3 entry visa there with a 6 month validity. Useing extensions and border runs you can get almost 9 months out of the visa by doing a border run just before the visa expires.

Or if you plan on staying here for a while then another option is to get a multiple entry non-o to visit freinds.