On revisits you will get half of the takeover points and 5 minutes of block time. Revisits can be done an unlimited number of times. (Provided a minimum of 23 hours passes between each visit and the zone is still yours.)
Revisits are not credited for most medals in Turf at present.
The Revisitor medal are given to one who makes 50 revisits within 5 days. If you log in at Warded there is a counter who help you count how many revisits you have done, eg "You have made 7 takeovers last 24 hours and 5 revisits the last 5 days." This can be useful when trying to get the Revisitor medal.