Depends where you leave the country at. Some of the Laos borders have been making people stay out for 2 days. If you use a visa run agency you won't have to stay out. If you fly out, you'll be fine too.
I did a run in December, it was cheaper to fly round trip to HKN than it was to take a van to Laos. might want to take a look right now at your options.
The 30 day extention is added to the end of your current permission to stay date, it doesn't matter when you apply for it. Phuket let's you apply 45 days before your expiration.