Hi there
Unfortunately there is no exact time frame, for some people, the improvement is almost immediate (within a day or two) for others its a couple of weeks, it really depends on how much inflammation you have and how well you respond to the steroids. For most people they notice a difference by about the week mark, so if it gets to two weeks and you haven't seen any improvements then it might be worth contacting your team. Some people need a slightly higher dose to get a good response.
As for the nausea, unfortunately this is something that happens quite frequently with steroids, sucking peppermint can be really good for dealing with this, and most people are put on medications to protect the stomach such as omeprazole which helps to stop any damage to the stomach lining and can help with things like nausea and heartburn so it might be worth asking for something like this. You can also ask for some antisickness tablets if its really bothering you and ginger is really good too. Hope this helps.