I still see lots of general misinformation around the speed of scaling in various missions, so this is purely for info/reference.
SP Void Cascade
When: 107 Exolizers
How long?
Approximately 1 hour in a team
Approximately 1.5 hours solo
The level scaling is determined by the number of Exolizers completed, not time spent in the mission, which means player performance affects the speed. Competent attacking to purge each Exolizer as it spawns, paired with competent defending to make sure no purged Exolizers are re-possessed, means a faster scaling mission.
Why: Thrax spawn more frequently beyond 16 Exolizers, which encourages longer runs for Arcane farming. Also for build stress tests, and fun.
SP Circuit
When: Round 18
How long?
Approximately 1.5 hours in a team - depends on player's loadouts
Approximately 2 hours solo - depends a lot on loadout
The level scaling is determined by the rounds completed, which means the quicker players get through the rounds, the quicker it scales. Clearing Void Flood, Exterminate, and Defense quickly, and successfully defending Excavators, makes a faster run.
Why: There are bonus points awarded at Round 4, and at Round 5 and beyond the points per round increase. This encourages longer runs. With more decrees collected making players much stronger there's also the chance that rounds go by much more quickly once you're further in. Also, for fun.
SP Disruption
When: Round 45
How long?
Approximately 2 hours in a team
Approximately 3 hours solo
The level scaling is determined by the rounds completed, which means player performance affects the speed. The faster players kill enemies, acquire conduit keys, put in said keys, and find and kill the Demolishers, the quicker the rounds go and the quicker it scales.
Why: Disruption can appear as a fissure, with scaling buffs to resources per relic opened (such as Steel Essence and tileset materials). This encourages longer runs. Also for build stress tests, and fun.
SP Survival
When: Approximately 8 hours
Survival, unlike the others listed above, does rely on time spent in the mission, as every 5 minutes counts as a "round" and this cannot be sped up.
Why: Like Disruption, Survival can appear as a fissure, with scaling buffs to resources per relic opened, which encourages longer runs. Also for build stress tests, and fun.
Level cap = level 9999 enemies
As a general rule, health/armour and damage reduction tanking falls off at around level 1000 enemies, as they start dealing too much damage for you to reliably tank through. You can generally expect to encounter level 1000 in half the time/rounds it takes to reach level cap in your chosen mission. There are some specialised damage reduction builds that can go further, and may hit different walls at 3000, 6000, or 9000.
How frequently are people reaching level cap?
You are generally unlikely to find players willing to commit the 8 hours it takes to reach level cap in a Survival. However, there will be many players willing to commit the 1-2 hours it takes to reach level cap in Void Cascade, Circuit, and Disruption.