In roguelikes, scumming is a strategy that consists of collecting easy rewards in unchallenging areas, e.g. when a high-level character visits levels suitable for low-level characters since they pose no risk but do offer some marginal rewards.
In Crawl, cheating by scumming was prevalent before version 0.6.0, especially using mummies due to their lack of a food clock: players would spend thousands of turns waiting on early levels for monsters to respawn, kill them without any risk, and thus raise their level beyond what was reasonable for that area. This rewarded tedious play behavior, and trivialized later branches since players entered them with a higher level (and power) than intended.
Since version 0.6.0, the game will detect when characters are attempting to scum and reward their efforts with progressively fewer monsters overall, with a larger percentage of them being out-of-depth monsters - think cyclops on D:1 after waiting a few thousand turns.
After more than 3,000 turns on a given level, the number of monsters that spawn will gradually decrease, until monsters stop spawning altogether after 15,000 turns. This is total turns on a level: you can't reset the counter by leaving and reentering a level.
Meanwhile, the probability of spawned monsters being out-of-depth will rise steadily with time spent on a level. These monsters can be from virtually any level below your current one, with a bias toward the current level + 13.
Finally, the game has a hard limit of about 200 million turns. If you reach this limit, your game ends with the message:
Outside, the world ends.
Sorry, but your quest for the Orb is now rather pointless.