How long does boric acid last when it is spread thoroughly to prevent roaches from reappearing in the kitchen?

I have just recently moved to a new apt. and the previous tenants were not very clean and they had roaches in the kitchen area. I had the exterminator spray and I put Combat all over the apt. as well as boric acid everywhere. It is working because I haven't seen any since I moved. I want to keep it this way. The boric acid seems to be the perfect protection but I don't know if it keeps its potency or does it fade away? I abhor roaches and I want a clean, comfortable apt. to live in.