My husband has been on Risperidone for a few years now. He had been having very extreme paranoid delusions and hallucinations. He was started on a very small dose of the drug, and it was gradually increased until they were brought under sufficient control. He's now on 150mg per day, and I very rarely hear anything about "them" - the invisible men who haunted him. From what I remember, it didn't take that long for it to start showing an effect. For him, it was in fact, as if someone had flipped a switch! There was an almost immediate very marked decrease in the delusions & hallucinations, and when the dose was increased, they stopped. After a while, the consultant tried to reduce the dose by a tiny amount, and the hallucinations and paranoia returned immediately. So the dose was reinstated. And my husband has never suffered any side effects from the medication, that I can see. I hope your mum will be as fortunate, and that it will have as good an effect. Anti psychotics are not to be recommended for the elderly at all, but when they are needed, they can be a god-send.
ps. If you don't start seeing any improvement, then do get back to the prescribing doctor. There are other medications that can be tried! Risperidone was the second one tried on my husband - the first one had no affect at all.