Which UX305? The UX305LA has an i7 or i5, the UX305FA has an old core M, the UX305CA has the latest core M. Some have HD or QHD displays.

EDIT: Mine is a UX305FA, the one with less powerful Core M processor. It's slightly slower than my Thinkpad x220, which has an older i7. So it's not a powerful machine, and I assume less powerful processor means better battery life than the faster x305's that have come out since I got mine.

I have been trying TLP but I failed to notice any significant difference in battery life... is it absolutely needed to tweak its default settings?

I don't remember. I would check power usage with powertop and make sure you've got no 'bad' indicators on the 'Tunables' tab (actually I think I still have one bad). On the 'Device Stats' tab you should be able to get 'system baseline power usage' under 5 watts under light loads. I run mine with display brightness fairly low, 20% or 30%, but it's still has comparable brightness to my other main laptop's (Lenovo x220) full brightness.

I actually have a Lenovo x220 as one of my machines, and with 70-80% brightness I get about 4 hours of battery life (wifi included) on a 9 cells battery. Does it sound reasonable or am I not getting what I should?

Sounds about right, I have 6 cell in my x220 (with Win7) and I'd guess its life is around 2.5 hours with those settings. It's short enough that I don't feel comfortable using it on the go, keep x220 at home almost always plugged in to AC. The battery difference between it and the UX305 is huge.

Thanks, good to know I'm in the average then. I wonder if the newer Lenovo (x250 for example) have much better battery life (on Linux) than the x220.

The x250 has great battery life. The keyboard is a little better than the X240, but not as good as the X220 or even the X230. Some of the keys are not full size. The function keys are garbage by default, but there's a persistent Fn lock (might be a BIOS setting, don't recall).

Performance wise it's very nice. Screen is decent, too. My only complaint is the that the keyboard has a bunch of non-full size keys. End and Insert are on a single key. Someone should be shot for that.

Hmm. My X220 has a 6-cell battery (almost new though) and I get 4+ hours easily with 70% brightness and medium workload (software dev).

I have the x220 with a used 9-cell battery (last full charge was 75%) and I'm easily able to get just under 6 hours of standard use (browsing, WiFi, watching movies, coding) and screen brightness at around 60%.