Pretty self-explanatory, just spend at least 3 hours on the DEVELOPER level.
The achievement is cumulative, so the time doesn’t have to be spent in one sitting. Depending on how long the Challenges take you to complete, this achievement could unlock over the course of your initial playthrough. If it does not, simply load up the level and stand around in it, you don’t have to do anything other than that for the time to count. As an alternative, you could check out the Museum area on the level to kill some time. It is unknown if pausing the game stops the clock, so if you are not in an active Challenge, do not press pause, just in case.
Unfortunately, there is no way to check your progress, so you’ll just have to be patient until it pops.
Special thanks to SATYATEX for his excellent tips that helped inform the ARENA guide in particular.