In general, I think it's fairly author/style dependant how much time you spend on descriptions. I personally like to give a brief overview of what a character looks like, focusing on what I feel is the most important details (for example, maybe the character has a tattoo which is relevant to them). I may then expand on that during a sex scene if necessary. That said, some might find my style light on the descriptive side. It may also be dependant on the situation. A person may analyze someone more deeply if they are interviewing them for a job rather than just checking them out at a party.
Considering Fanfics feature known characters, I'd steer towards being light on descriptions, as people would be familiar with the character. Despite the fact they are known, character descriptions are most likely a useful tool (DISCLAIMER: I haven't written any fanfic), you may have to differentiate between certain continuities (For example, a character in the Marvel Comics may look different in the Marvel live action movies). Character descriptions may also be important if writing from a first-person perspective. In the end, I'd say so long it doesn't bog down the pacing too much, any description is fine.
The only thing that I really dislike is when people use terms such as 'he's five foot eleven' or 'she has 34D breasts'. To me it reads more like a police line up (which to be fair could be fine if the context of the story is a police line up) rather than a person looking at another person.
