Time has nothing to do with it. What matters is that you not be an emotional retard. Friends who appreciate each other can often have better sexual relationships than a dating couple, because there are no hidden catches or expectations - you're sleeping together because you feel close to your friend and you want to share something more intense - not "deeper" or "special" (
). The only time this doesn't work is when you think you have to have emotional attachment to everyone you sleep with, and feel that you have to complicate the situation with shit like "oooh what's it going to be like when we wake up?" Fuck off. Tommorrow has nothing to do with it, you're making love now because you appreciate your friend. You KNOW your friend is going to remain your friend and that what you're doing is in the interests of the friendship. You have none of this shit about dumping or self-sacrifice or betrayal because that shit doesn't exist in friendships. That's why friendships last pretty much forever whereas most people aren't capable of holding a relationship together for more than a couple of years.
Ever think about how absurd it would be to suggest that madly-in-love married couples shouldn't sleep together because it might ruin their relationship? Good friends are in exactly the same situation, minus the feeling of being crazily in-love. If you can't handle that, then you probably shouldn't be going out in public in the first place.. better to stay at home and socialise with the cockroaches, you might even learn a thing or two.
As for friends -> lovers, I disagree that this is any harder. In fact, if you've been friends for ages and then one day realise that there's more there, your chances for creating something spectacular that lasts are much higher - you already know the person inside-out, and you already know that you get on great together. Friends -> Lovers is in fact the IDEAL way to start a relationship. And because it's your friend, if finally it doesn't work out you know each other well enough to accept it and go back to being friends again.
What are you, a catholic priest? Honestly, that shit made me sick.