It's still very early days. You won't get an erection just by taking it - you need to get sexually aroused too. Sildenafil works best on an empty stomach, and also when you haven't eaten fats/oils. (For Tadalafil, it doesn't matter if/when/what you've eaten.)
Also, if you didn't get any unacceptable side effects from it, you should ask for the max dose, 100mg, at this stage.
In terms of saying if a PDE5 inhibitor is working or not, you have to try it 4-8 times before you can say it doesn't work as they don't necessarily work every time. Obviously, stop before then if it generates unacceptable side effects.