I've never tasted imitation maple flavor, but I can tell you with certain there will be a significant difference in flavor between imitation maple and the real thing.
I've also never used maple extract, but I have used maple sugar and maple syrup. In the sugar and syrup format, which I assume to be far less concentrated than extract, you need to use a large amount to get anything more than a subtle flavor.
If you're kegging, I would start with adding a small amount and tasting the beer to see the impact. Do this until it reaches the level you want.
The other option is to dose a measured pour of beer with a measured amount of extract until you find the ratio you like and then scale up the volumes.