The snake may defecate as rarely as once a month or as frequently as once a week. A lot depends on how frequently it is fed, how old the snake is (younger snakes feed and metabolize faster, thus make a mess sooner) and humidity. If the snake is dehydrated you can end up with retention problems. Normally the best way to facilitate a bowel movement is a good long soak in 82-85 degree water. (Fill it only about half as high as the thickest part of the snake to ensure it doesn't drown.) Try to soak it the night before the vet visit and you might get lucky. Was it regularly feeding before last monday? If so, can you find out when it last defecated? That should give you a good indication of when it will again, if the prior owner kept decent records.
All that said, I would still take it to the vet, poop or no poop, and later, when it does defecate, you can just bag the sample (refridgerate if it will be over a day) and bring it in. The vet doesn't need the animal in front of it to prescribe standard anti-parasite treatments.
Good luck.
-SWA