Is it okay to feed my cat with regular cat food?

About 3 years ago my cat was diagnosed with urinal tract infection, the vet successfully cured him, and strictly stated that I should not feed him with any other cat food than "Urinary S/O". Which is very expensive compared to regular commercial cat foods. At the moment im short of money, I still could by that food, but I need the money for other things. So how big is the risk that my cat will get sick again if I feed him with regular cat food, for lets say a month.