I think there was a bit of a misunderstanding about what the demo was suposed to be showing. The demo showed someone spawning a load of guns (more than you could buy with $16000) at the start of the round, but the player was not hacking (aimbot/walls/speedhack etc.) so got the "not enough proof" reply. I've replied to your "Raining Guns" thread about this.
It's not always how long it is Hitman, it's what you do with it
. To show someone is cheating the amount of proof you need will vary depending on the cheat and how obvious they are being about it.
Speedhacks don't need very long demos, as long as the demo shows the players name i.e. when they get a kill or die.
Aimbots. It's not so much the length of the demo but the number of times the player fires at the opposition, a 30second demo of someone killing 10 is far better than a 10minute demo of someone who only fires their gun once.
Wallhacks. Often harder to prove so will need to be longer or several demos, depends on how obvious the player in the demo is being.
When I record demos I record for 1 or 2 rounds, then stop and record another. Then I watch them and pick the demos that best show the player cheating. If I'm watching some demos of someone I think wallhacks I'll type r_drawothermodels 2 in console, it lets you see all the moveable objects through walls.