The first time I took creatine (6g IIRC), I actually felt the mental effect just 2 minutes later. A pleasurable sensation of augmented presence and (mental) relaxation.
I paused taking that momentarily out of precaution while I wait some physical issue to normalize, but I plan to resume it in some weeks. Also it is considered a very safe supplement.
How can you be sure you weren't placebo'ing yourself into feeling it?
To my knowledge creatine has no significant effects until your levels rise after, say, a week of taking daily
Well.. I can't, nobody can. Perhaps it was a placebo effect or perhaps I had low levels of creatine and felt the difference. I often felt better right after taking it though, not consistently but often.
I've felt dehydrated while I'm in loading phase, it's very noticeable from that perspective. But the mental effects only come after at least a week (for me: longer attention span, less impacted by poor sleep).