The degassing is mainly done to remove the adsorbed species from the sample. Actually, BET work with the formation of N2 layer on the surface of samples which could be monolayer or multi layer.
this is an article which carried out for measurement of Degassing various forms of activated carbon doi: 10.1016/j.mspro.2014.07.033
To select degassing conditions for different materials you need to know the peculiarities of your samples.
1. If the material has low thermal stability you can use only low temperatures and a vacuum for degassing.
2. If the material's porous structure consists of channels with of microporous dimension. You need to degas it for a long time period.
3. In general you must take to attention literature data concerning similar material investigations.
Lyubov's answer is quite good, so I will just try to complement it.
In general, it is material dependent. Porous carbons you can do it overnight at 200 degC. Inorganics such as alumina and others may require higher temperature for a better cleaning (they are hydrophilic so they tend to adsorb moisture strongly). In general you should try to go at some moderately high temperature, but avoiding decomposition of functional groups or the adsorbent itself.
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