not fixed. it varies depending on demand.(its a bidding war concept.. like an auction)
https://bitcoinfees.21.co/  helps find a good estimate.

the green line tells you the biggest chance of getting into the next block. at the moment it is 70sats a byte
which if you are just doing a 1in 2out is ~15820sats fee

some people get lazy and just put a fixed amount of say 80sats a byte knowing it wil most of the time outbid everyone to get in the next block.. only problem is if everyone done that. you then have to increase it.. (its a bidding war concept)