The most known rule seems to be that you should never sleep with a date until at least the third date. There's no real logic but it does mean that you get a little breathing space before you start feeling that you 'should' be having sex! The general idea is that you should never have sex on a first date because that's going to get you a reputation for being 'easy'!
General consensus and opinions aside, whether or not you have sex on your first, third or tenth date is only up to you and your partner! Only you know if you are comfortable enough with your date and ready to have sex with him/her. No-one else really has the right to tell you that it's too early and it's not the done thing, or for that matter to pressure you into something that you're not ready for when you've had a dozen dates with your partner and haven't got past a chaste kiss on the cheek! Have control of the situation and decide it by yourself!
The worst situation with dating and sex is when there is an expectation that your date has created. They are clearly anticipating an intimate experience with you during or after the date. If you're up for it, that's great. If you're not, then you need to get this out in the open as soon as possible and give your date a chance to find another bed partner for the night if that's all they're looking for.
Don't feel guilty or uncomfortable when you say 'no' concerning sex to your date. Honesty now is better than getting them angry and feeling that you led them on later in the evening. It's also better feeling guilty with them than feeling bad about yourself because you followed their expectations and did something that you didn't want to do.
Listen to your mind and your body. They will tell you when the time is right to take your relationship to the next level. This could very well be within the first couple of dates if the chemistry is right and your intuition is telling you that your date is not just in the relationship for sex.
On the other hand, it could take some time, especially if you've had a bad experience in the past after having sex with a date early on in the relationship, before you're ready. Remember, there's only one person's opinion on when sex becomes an option within the relationship, and that's yours!