The purpose of decanting is to aerate the wine so its aromas and flavors become more expressive and to separate the sediments that may have formed.
I rarely decant the Brunello at home and most of the time I prefer to leave it in its open bottle for several hours. I will likely have a sip from time to time to see how it goes if I am around. I've found that 4 hours is the best time for a Brunello to breathe in its bottle before serving, but again it depends on the age of the worthy liquid. The older the wine the longer time it may need.
There are some circumstances however where you better call for a decanter, for example if some friends call round unexpectedly or you feel like you really want THAT particular Brunello tonight and you did not have time to let it breathe before. Decanting would accelerate the process of making your wine arrive at its best. From 30 to 90 minutes should be sufficient. You can even speed up the process by swirling the wine in the decanter. Note that if you have a very old vintage it is better not to overdo it and I would advise no more than 30 minutes, otherwise the Brunello may become oxidized.
If decanting serves the purpose of separating the sediments, remember to keep the bottle in a vertical position for about 24 hours before opening it so the sediments have the time to settle down. While you are pouring your wine have a candle below its neck so as to see when the sediments start.
In conclusion I personally prefer not to decant as I like to see how the wine evolves in my glass. It can be like having an interesting conversation together!
* Wine featured is a Brunello 2008 single vineyard "Vigna Fonte Lattaia" from Terralsole. Decanted about an hour before we started to taste it, it continued its evolution over dinner. *