This isn’t entirely true, but 50% and 100% are usually safe numbers due to the following:
All conditions that deal damage over time (except confusion) will tick damage every WHOLE second.
For example if you have a 5.25 second burning that does 500 damage every tick:
At 0 seconds – the burn is applied
At 1 seconds – you take the first 500 damage tick from burning
At 2 seconds – you take the second 500 damage tick from burning
At 3 seconds – you take the third 500 damage tick from burning
At 4 seconds – you take the fourth 500 damage tick from burning
At 5 seconds – you take the fifth 500 damage tick from burning
At 5.25 seconds – the burning ends.
The bottom line is that conditions do damage in one second intervals so in order to maximize damage you always want your condition’s duration to be a whole number. That extra 0.25, 0.5, or .75 of a second will not give you any additional damage.
So the reason that people say 50% and 100% are magic numbers is because most (if not all) skills that apply a damage condition have a base duration of a whole number (3s poison, 4s bleed, etc). So no matter what, if you have 100% duration, you’ll double the duration, which will always be a whole number. The 50% only works for condi durations with a base duration that is an even whole number.
The thing that you need to know is always aim for the next full second when determining condition duration on damage-over-time conditions and when calculating duration remember that GW2 always rounds to the nearest 1/4 of a second.
For conditions that act as debuffs (cripple, weakness, etc) more duration is always better, because the effect is not ticking but rather a continuous, so a 5.75s cripple will be better than a 5.0s cripple.
This is the most basic description of it and is really all you need to worry about. In bigger fights when there are multiple sources of non-stacking conditions (burning, poison) this idea is flawed due to the timing based on the first application, but that’s really something more for theorycrafting rather than actual playing.
Vent server is available for member use. Representation would only be required when you’re in WvW on your thief. Let me know if you would be interested!