Thailand and Laos have different rules for bringing in bikes registered in other countries. If you bring it into Thailand, your bike gets a 30 day "tourist visa" - a temporary import permit. After that time, you are legally requred to take it out of country. Some people keep it in longer, and have to pay overstay penalties when they leave. Might be a problem if you have an accident with the bike in the country illegally.
Laos used to give only 7 day temporary import permission. And the first day is the day you enter, so you had to leave 6 days later. I have heard that they now give you a longer time, but cannot personally confirm.
It also depends where you exit Laos. I left once via the Freindship Bridge, one day late. Had to pay 200 baht overstay. I have exited twice at Chong Mek - Southern Laos, near Pakse. Both times late, both times got stamped out with no penalty.
You are also required to have temporary Lao insurance. The crossing from Cambodia is pretty primitive, and I am not sure of availability. I have always purchased it at the Friendship Bridge, $3 US for 10 days coverage.
FYI, the road in Cambodia to that crossing is pretty bad. Last year met a rider on a KTM 640 adventure, who had just come in. He reported that the last 50 KM in Cambodia took him over 3 hours to ride. You also have to balance the bike on a platform between 2 small boats when you cross.