The date you decide to marry your Lao girlfriend depends on the situation so there really isn't a set amount of time, it's whenever the both of you are committed to each other and feel it's the right time.
If she was your best friend for years then maybe a short courtship would work. On the other hand, if you just met her and started dating I would give it at least a year of dating to get to know each other better.
You also need time to get to know her parents and the rest of her family because Lao girls in Laos are very close to their family. Therefore when you marry her, you inherit the whole family. So you need to get along well with her family because a Lao wedding wouldn't be possible without the support of her parents.
The time it takes to complete the marriage paperwork also needs to be taken into consideration if you plan to get married in Laos. It could take up to a year for Lao authorities to go through your documents, then they need to give their approval before a wedding can occur.
Sometimes Lao couples marry early due to peer pressure in Laos. In the old days and still occurring today, peer pressure makes an influence on couples decisions to get married especially if the girl is pregnant. In Lao culture, it is frowned upon heavily by the Lao community and an embarrassment to her parents if their unmarried daughter has a child. So an early wedding is required to save face before the baby arrives.
If you don't know much about Lao culture then try to learn more about it from your Lao girlfriend and it will set you up for one of the most fulfilling relationships ever.
Lao girls are beautiful, gentle, kind, and they make good partners and even better mothers. She will take care of you well into old age. Just make sure, before you propose to your Lao girl that you're sure you're doing it for the right reasons and not just swept away because she's so damn gorgeous.