The each function in jQuery already handles an index over iteration with the first parameter (https://api.jquery.com/each/). I modified your code to handle it. You were not really specific with what you wanted, but the handles an increase of 50 ms at each pass, the first pass being at 250 ms of delay.
var rfi = 250;
$('.animate_rfi').each(function(i) {
$(this).appear(function() {
$(this).delay(rfi + (i * 50)).animate({
opacity : 1,
right : "0px"
}, 1500);
});
//rfi += 50;
});