It depends on a lot of things, like how old you are, how much exercise you get, when the party is, what dose of Adderall, whether or not you eat during the day, your resting blood pressure, how hot the chick is, other drugs you might take, how relaxed you are when you need to get it on, etc.
Typically people don't get stimdick until you do large amounts of amphetamines, like 30-50mg or more. And it's definitely different for everyone, some people can get a boner on 120mg of MDMA, some people get soft on a 20mg adderall.
If you're young and healthy and only take one dose of your prescribed adderall in the morning and then go out to party at 6 or 8 PM you should be just fine.