I believe the numeric value will be treated as seconds if dealing with timestamp type of field, but am not 100% sure without testing it first. However, it would probably be safer (and easier to maintain) if you used INTERVAL to make it explicit:
NOW() - INTERVAL 200 SECOND
...or...
NOW() - INTERVAL 200 MINUTE
...etc....
PS: I'm assuming MySQL here--this may not work with other DBMS's.