Tamsulosin kicks in relatively quickly, around 24 to 48 hours and the dizziness is attributed to it reducing BP in some people.
I've been on this med for over 8 years and have never experienced any dizziness during that period.
One word of warning and that is watch out for the side effect that causes the muscles controlling the irises to become "floppy".
It can cause problems for the surgeon when performing a cataract operation and rarely as in my case caused me to develop pinpoint pupils which can lead to some people believing you taking recreational drugs but from an eyesight perspective, it doesn't cause me any problems.
However, this side effect only affects one in ten thousand (aren't I the lucky one), so I wouldn't worry about it.