How can I get THIS colour :( ?

No, I'm not saying a direct dye won't fade. I'm saying, it will fade, but you can top it up often with no damage. You can't do that when using permanent dyes. But the permanent dyes still fade, as you've found out. Why use something, often expensive, that damages quite heavily when you can use something cheaper that lasts almost as long and doesn't damage.

There's no set time for how long you leave the direct dye (with or without conditioner) as it's totally non damaging. Again though, until we know what resultyou actually get from bleaching or whatever method you choose, then it's difficult to predict. It might not be that you actually need blue. You might just need purple, or something in between.

Hello! This is what I finally did. I bleached my hair (30vol. developer / 40 minutes) and then I dyed it using Revlon Colorsilk 60 (Dark Ash Blonde, which is ammonia free). Well... the result is a nice colour, but honestly, I think it is too light, especially under natural light, in which it looks kind of a pale yellow. I don't think it really suits me, and I think it should have turned darker, considering it's DARK ash blonde. So I decided that the next step should be trying a Light Ash Brown 5.1 in order to fix it, but I need your advice. What do you think? I attached some pictures of my hair after bleaching, the dye I used and the final result.