PKU is a genetic disorder that your mother would have inherited from both of her parents (both were carriers and probably didn't know it until your mom was born). With PKU your mom should be following a low protein diet and usually people with PKU will also take in a special low protein formula. There are a few websites that have really good websites with recipes such as cookforlove.org, and dietforlive.com might have some. As far as how much protein your mom can have would depend on the severity of her mutation. Everybody is different and there isn't a set amount that everyone can have.
Technically what happens in someone who has PKU is that their body cannot breakdown phenylalanine. Usually it breaks down into tyrosine, but with someone who has PKU they can lack all or some of the enzyme that does this. When they lack it either partially or in full then the phenylalanine overflows into the blood stream and can be deposited in the major organs and cause issues over time.
As far as something bad happening quickly that isn't how PKU works, but instead it is something that can go slowly over time (with babies and young children it affects the brain rather quickly, but with adults it can happen slower). Being untreated as an adult can cause serious complications over time especially if your mom has one of the more severe forms.
I am curious – why won't she talk to you about PKU? After all if she has PKU then you would be a carrier of the disorder which would mean you could pass that gene onto your babies someday. They would only have PKU if your partner was a carrier or had PKU as well.