"It hasn't been long enough yet!" - Just like I told a good friend of mine recently, don't put a time stamp on when you can and can't say "I love you." Suppressing your feelings--at any time--isn't the best thing to do, and you'll wind up getting in your own head and potentially become self-sabotaging. Now, I'm not advising anyone on a first date to go and say "Hey, thanks for taking me out tonight--I'm in love with you!" I am saying if you truly feel like you love that person, tell them. If it's been a couple weeks, a month, two months, four months, whatever, tell them! You are presumably comfortable enough with them already to have these feelings, so be comfortable enough to share how you really feel. Trust yourself.
"What if I say say I love you, and he/she doesn't say it back?" - Well, that's okay! Hearing something like "thank you!" after you've told someone you love them may not necessarily be the response you'd like to hear, but simply remember that again, you were being honest with how you were feeling, and that every person is different. Some people may be more comfortable saying it than others; it doesn't mean their feelings aren't reciprocated, they just may need a bit more time to process and express it.
"I don't want to be the first to say it." - Don't play games--that's about all I can say here. I understand, being a female, that you may want your partner to be the first person to share his/her feeling with you, but I'll revert back to "It hasn't been long enough yet" and say get out of your own head. Express how you feel, because you have nothing to lose when you are being honest with yourself and your feelings. If anything, you'll probably relieve some of the pressure your partner feels, and make it easier for them to say it back to you.
I will say though, I'm not sure you'd want to tell someone you love them for the first time when you are drunk. Make sure the person knows it is you speaking, and not the 3 vodka sodas. I'd probably steer clear of telling someone you love them for the first time while being intimate, too. It can be easy to get caught up in the moment and blurt out "I love you!"--but the recipient may wonder if he/she loves them, or loves the orgasm. Finally, I'd recommend saying it face-to-face. I know text messages and social media outlets make it easier to avoid face-time nowadays, but throwing an "I love you" at someone for the first time via text message with a bunch of respective heart emojis probably isn't the best way to go either. Just sayin'.