Let’s start with some background. The gemara[1] asks about the significance of the baraita’s statement that the Chanuka lights be lit “ad shetichleh regel min hashuk” (“until people no longer are walking around in the market”). The gemara gives two possible explanations: 1. One must light before that time. 2. One should put in enough oil to last until then.
The Rambam[2] and Rosh[3] determine that the amount of time from the setting of the sun (when the mitzva begins) until tichleh regel min hashuk is approximately half an hour. The Shulchan Aruch[4] rules that one is allowed to extinguish the flame half an hour after lighting the candles. Certainly one need not put in enough oil to last longer than that minimum time, and the Mishna Berura[5] concludes that there is not even a hidur mitzva (religious preference) to do so.
We pasken that one who did not light by the classical tichleh regel min hashuk can do so significantly later because: 1. Only one explanation in the gemara[6] gives an end point for lighting.[7] 2. Nowadays, people are out on the streets later. 3. Nowadays, the main mitzva is to “publicize the miracle” inside our homes.[8]
The question is why this halacha of having the light last for half an hour does not also change with the times. The Bach[9] points out that there seems to be a machloket in the gemara whether there is a minimum amount of oil needed and, thus, there may not be a line of demarcation after which it becomes proper to extinguish the flame and/or make use the light. Following this logic, it would seem that the longer they stay lit, the better. Mikraei Kodesh[10] brings such an opinion in the name of Rav Mordechai Eliyahu and others. According to the Mishna Berura, we must assume that although Chazal instituted the time period of half an hour based on their situation, we have no source to mandate more.
Another issue is how to count the half hour. Some feel that the setting of the sun refers to astronomical sunset. Others say that it begins at tzeit hakochavim (when the stars are visible), and many minhagim exist. If one does not follow a certain halachic approach unequivocally, then it would be appropriate that the candles remain lit until half an hour after tzeit hakochavim.
In summary, half an hour of oil or candle is sufficient. Preferably, it should last until half an hour after tzeit hakochavim, and some prefer significantly longer lighting. Although it is permitted to extinguish the flame, it might be better to put in a little less oil on a night that you expect to leave the house.