They will chase you down to the ends of the earth, or at least where an NPC can chase you to. There's no activation needed, the second it's installed they will start the hunt. There are three groups out to get you, the Courier:
1) Hellcat Company: a mysterious organization that's settled in the Mojave's shadows amidst the conflict between the NCR and Legion to work for New Vegas crime families and anyone willing to pay for their fees. They're power armored and energy weapon geared. 
2) Jade Company: originating from the west, Jade Company works under the orders of a figure known as The Senator, whatever you've done, knowingly or not, you've pissed someone off and they've sent assassins to "remove" the problem. Disguised and armed with common ballistics, they're in and out without a track. 
3) Bad Luck Wolves: a new group building themselves up north and moving south, their funds have hit a wall and so they've poked their heads in the bounty market where they found you, unfortunately. Led by a rumored former Desert Ranger, they're supposedly the best of the best, on par with the original Rangers, and they don't leave a job half-assed. 

Lucky's Note:
-An easier version is in the works, not consisting of the groups above but of smaller tier enemies, less groups and more single parties. 
We have a Youtube channel where I use my own mods to showcase them alongside reviewing others. Check it out if you want, or don't, I'm just text.