Originally Posted by tangoll

I think you may be confusing visa with permitted period of stay upon entry. For US passport, you are exempt from having a visa upon entry into Thailand, and at immigration you will be granted entry with a 30 day period of stay. If you leave within this 30 day period, everything is fine, and if you re-enter at a later date, you will be granted a new 30 day period of stay.

As you will be travelling to different countries, presumably in and out of Thailand several times (?), each time you enter Thailand, you get a new 30 day period of stay, and hopefully, your last period of stay is sufficient to overlap the date of your return flight to your original departure point.

I don't know if French passport holders get the same 30 day period of stay, but I imagine it would be similar to US passport holders.

If you do get a visa, and it is granted, there is a visa validity period (probably 90 days) which means that to use the visa for entry, you must enter within the visa validity period, or else the visa expires and cannot used for entry. That visa validity period is not the same thing as the period of permitted stay upon entry.