Hi, welcome to your new addiction (Nepenthes), bwa ha ha!
I fill my pitchers to the 1/2 way point whenever I receive new plants. It helps the plant rehydrate after it's trip. I wouldn't suggest feeding these pitchers because the water you add is not the same as the digestive fluid produced by the plant. If bugs get in on their own I wouldn't worry but don't intentionally feed these older pitchers. When a new pitcher opens after it's been in your care give the plant a smushed cricket or Super Worm from the reptile store (both can be bought live and then frozen for use whenever needed).
A lot of people think they should continually "top up" their pitchers but it just messes up the balance of acids and enzymes inside the pitcher fluid. Your pitchers will naturally remain about 1/6th full, if water gets in while you're doing some overhead watering don't worry but don't intentionally fill them up to the peristome lip as the plant may not be able to properly digest the pitcher contents and this can lead to rotting or fungus in the pitcher. I will add that there are some species which benefit from being refilled: N. ampullaria, N. inermis and perhaps N. dubia, N. lowii, N. epippiata. these pitchers all have lids which are very tiny and held reflexed back and do not overhang the pitchers as lids do on other species.
Pitcher lifespans depends upon the species biology and in part growing conditions. Pitcher lifespans can be anywhere from about 40 days to almost a year if they are not excessively fed or filled with water. I have one plant that would make pitchers which died off rather quickly so I moved it to a cooler growing chamber a year ago and haven't had a pitcher turn brown yet! I don't feed these old pitchers but they still have not shriveled.
Be sure to give your plants high humidity 85-100% and bright lighting - with real good lighting (natural sun or at least 4 x 40 watt flourescents) you can have pitchers as big (or bigger) than each leaf until the plants starts to mature-depending upon the species. Temps are all important too, do you know if your new plants are Highland species (cool growing 50-60*F at night 70-80*F during the day) or Lowland species (warm growing 70*F minimum at night 80-100*F during the day)? If you don't know someone here will be glad to tell you.
If you have never grown any CPs or orchids before you also need to know that you should not use any water other than reverse osmosis, distilled or rain water that you collect in a clean pail. CPs are completely intolerant of salts or minerals in their water. Since they do not take up a large amount of nutrients from their roots (that's what traps are for) minerals that get to the roots tend to stay there and have a tendancy to dehydrate the plant. Minerals and salts crystalize and block the roots from absorbing water and the plant litterally dies of thirst even though you keep pouring water on it.
hope this helped some!