There is a short acting SSRI that has been effective in clinical settings going by the brand name priligy (chemical name dapoextine. It's short acting, and works within 45 minutes, with few side effects. It's definitely worth a try if you have access to it.
If you're looking to increase your sexual endurance naturally I'd suggest doing a reverse kegel routine, edging, and doing breathing exercises. Learning your body's responses to sexual stimulation and controlling sexual arousal through edging may also help. Stretches and yoga can help loosen or correct pelvic floor muscles which might otherwise be tight or overly lax.
That being said, tramadol has also been used to effect in clinical settings to increase the duration of sex, so it's also an option, but it's not a silver bullet by any means.
If you're able to continue a short while after ejaculating you can try and keep going (viagra would help with this, but don't take it with anything else).
Alcohol can, on occassion, inhibit orgasm as well.
If you're with a steady partner, you can discuss other ways of getting off together that don't involve penetrative sex. Or you can try a stop-start method, where, when you feel you're reaching the point of no return, you grab the tip of your penis and hold it quite firmly and wait for the feeling to subside. Obviously agree with your partner about this beforehand otherwise they might get a bit confused and frustrated.
If you want more info please PM me.
Prozac would work right away for this, or does it need to build up?
You'd need to build up the effects. Prozac's primary effects as well as it's side effects take a month of daily usage to start showing.