How often should I be paying for my girlfriend when we go on dates?

I believe the 'treats/buy her stuff' should be mutual, so if should be split.

And for me, it's not about the dollar amount that goes into treats and presents, but more of the thought behind it.

In terms of frequency (since you mention how often) - if you get dinner, then maybe she can get dinner the next time. Unless it's a special occasion like her birthday, then probably you would pay for the full sum for the meal. Likewise with presents.

In terms of dollar amount (since you mention how much is enough) - it really depends on your budget. And how much she wants expensive meals/expensive presents.