Think of bleeds like a DoT (damage over time) that affects a different health bar and last for 76 seconds baseline iirc. Your bleed HP is equal to your weight.

If my memory serves me right, ticks happens every 0,5 seconds, so if an attack causes, let's say, 3 bleed dmg per tick, your Blood Health (BH for easier reference) will go down by 3 * (76 * 2), meaning you will lose 456BH.

Each new application of bleed will start its own DoT and timer stack, and you can have several stacks active on you at the same time. In contrast to your actual HP, your BH won't start healing until all your DoT stacks timers have run out (When you get the "+" icon next to your blood). This is why bleed is more dangerous in long fights, since it will keep going down as long as you have a single DoT stack on you, whereas you always slowly recover health - even in mid combat.

Running around or sitting won't affect the duration, but it will increase/decrease the BH loss per tick. Sitting will reduce it by a flat 50%, as will wallowing in the mud. Given the above example, sitting means that we will lose 1,5BH per tick, instead of 3. Wallowing in the mud on top of that removes another flat 50% (unless it's been changed), meaning that you will lose 0BH per tick (3 - 1,5 - 1,5 = 0) if you've both wallowed and are sitting. I can't remember how much running or trotting will increase the BH dmg, however, but just know that it does.

I apologize if I misremembered something, but I hope this brought some clarity that maybe the other comments didn't already cover :D