I have two kids with PKU. I nursed both of them until they didn't want to anymore (my son @ 4 months, daughter @ 6 months). The feeding schedule was similar to two bottle feedings with formula for every one breast feeding. Eventually they wanted the bottle rather than nursing because it is easier. After that I would add regular baby formula to their PKU formula (their dietian figured out how much of each) and bottle fed them. I didn't start baby foods until they showed interest in eating it. Hope this is helpful.
I wouldn't do that personally- my little one is almost 10 months old and in talking to my dietician we are trying to keep her nursing as long as possible- She obviously doesn't get to nurse often – she takes 3 bottles a day of formula and is now taking solid foods but even 50ml of breastmilk is enough to help build immunity and it seems pku kids get sicker more often! So our goal is to continue at least 50ml a day of breastmilk (just under 2 ounces) until she is 4.
To me porridge would NOT be an adequate replacement! at 6 months we started introducing solids- including pku special low protein baby pablum (like porridge) and then fruits and veggies- and decreased the nursing but it is so very valuable i'm not ready to cut it out entirely!!!!!