I'd say take your time, keep on trying the demo, and if you are still playing LFS in, let's say, 3 months, then you're ready to make the right decision about that.
It took me 1,5 years of demo racing to realise, that everytime I got bored from LFS and put it aside, I always returned to it after some while. So I purchased the S2 license, and yeah, i'm still here.