If you're just checking her out, I usually go with a 2-3 drink limit. This would put you at about the 90 minute mark in the date. Past that you might get bored or run out of things to say.
If you're trying to bang, which you should be trying to do generally, you need to hit your sweet spot. The sweet spot is generally the amount of drinks needed to be as social as possible while remaining sharp and strategic. I have a routine where I drink two glasses of whiskey and then switch to to watery light beer the rest of the night. The whiskey insures that I'm loose and social. The light beer helps to mask the fact that I'm not really drinking much. It's equivalent to ordering a coke because the alcohol content is low and it takes longer to consume. You can sip the light beer at that point to slow yourself down; meanwhile continue to order her gin and tonics. Also it avoids any awkward conversations about why you're not drinking and encourages your date to drink by applying social pressure (another round?). Switching to light beer also helps me keep my focus and I don't wake up with a hangover the next day. Anyways, that's my method.
If you're trying to bang, which you should be trying to do generally, you need to hit your sweet spot. The sweet spot is generally the amount of drinks needed to be as social as possible while remaining sharp and strategic. I have a routine where I drink two glasses of whiskey and then switch to to watery light beer the rest of the night. The whiskey insures that I'm loose and social. The light beer helps to mask the fact that I'm not really drinking much. It's equivalent to ordering a coke because the alcohol content is low and it takes longer to consume. You can sip the light beer at that point to slow yourself down; meanwhile continue to order her gin and tonics. Also it avoids any awkward conversations about why you're not drinking and encourages your date to drink by applying social pressure (another round?). Switching to light beer also helps me keep my focus and I don't wake up with a hangover the next day. Anyways, that's my method.