Laos in 2-3 days
If you’ve only got a couple of days to devote to a visit to Laos, your best bet will be to fly direct into Luang Prabang and out again from the same destination. There’s easily enough in Luang Prabang to fill three days, and there are several interesting destinations out of town that make great day trips.
Laos in 7-9 days
A week is long enough to take in a night or two in Vientiane, two to three days in Vang Vieng, a couple of nights in Luang Prabang then a two-day cruise up the Mekong to Huay Xai, from where you can head into Thailand.
Laos in 2 weeks +
You can stick to the route above, adding extra time in Vang Vieng and Luang Prabang. There’s enough to do in and around these times to easily fill in a couple of weeks.
Heading further afield will certainly be rewarding, but allow yourself plenty of time. Laos isn’t the sort of place suited to go-go-go travelling where you hop to a new destination every day.