Something like

var mouseStillDown = false;

$(document).mousedown(function(event) {
    mouseStillDown = true;
    doSomething();
});

function doSomething() {
    if (!mouseStillDown) { return; } // we could have come back from
                                     // SetInterval and the mouse is no longer down
    // do something

    if (mouseStillDown) { setInterval("doSomething", 100); }
}

$(document).mouseup(function(event) {
    mouseStillDown = false;
});

var int00; // declared here to make it visible to clearInterval.

$('#trigger').mousedown(function(){
    int00 = setInterval(function() { repeatingfunction(); }, 50);
}).mouseup(function() {
    clearInterval(int00);
});

function repeatingfunction() {
    // This will repeat //
}

You can also put a clearInterval on the mouseleave event.

You'd implement some recursion!

var mouseisdown = false;

$(document).mousedown(function(event) {
    mouseisdown = true;
    doSomething();
}).mouseup(function(event) {
    mouseisdown = false;
});

function doSomething(){
    //Code goes here
    if (mouseisdown)
        doSomething();
}