I hadn’t realized that you’ve bought an egg. I would just wait patiently and check the egg often to make sure the embryo is still alive. You can hold up the egg so that light shines through it to see if he’s alive and moving. Be careful not to lift the egg out of the water as you can get air inside of the case. Also check for fungus or rotten smell.
Depending on temperature and how developed the egg was when you bought it, the pup can hatch anywhere from tomorrow to a few months. The incubation period for bamboos is, I think, around 5 months or so. The best way to see when it is about to hatch is to look at the developing pup relative to the size of the case. When he does hatch, which will probably be over night, try to resist helping him out. But if it seems as though he stuck, you can enlarge the hole on his egg case a little with a pair of scissors.
Lastly, the pup will probably not eat until a couple of days after hatching. So don’t be discouraged if he doesn’t eat right away.