That's a tough question to answer. After 2-3 days you might not see improvement but if there is any worsening of their condition it's not working. Holding steady rather than declining is a good sign at this point. I would expect to see some sign of improvement, like less stringy saliva over the next few days. I haven't used cefalosporins like Ceftaziamine with mine so I don't know how long or often its given or exactly what to expect.
It sounds like you are getting injections. Be sure they are given in the front half of the body, reptile circulation can eliminate drugs given in the back half of the body before they reach full circulation rendering them ineffective.