Load model for gatling with fixed number of users

HI Team, what is the setup config for the below scenario.

I have to run the simulation for 3 hours with fixed number of 25 users.

Note: I do not want to kick off the new users everytime.

If you don't want any of your users to leave the system. You probably would have to use the 'rampUsers/atOnceUser' function along with a 'during' loop to iterate over the scenario for 25mins