How to get how many minutes the user is idle by using user lastActivity?

Im using PhpMyadmin Mysql

this is my sql record from database, How can I use timediff to subtract NOW(); from last activity which is user is user1 and get how many minutes user1 is idle.

id   lastActivity           user
1    2015-05-08 00:20:40    user1
2    2015-05-08 00:32:29    user2