If you want to set something on a timer, you can use JavaScript's setTimeout or setInterval methods:
setTimeout ( expression, timeout );
setInterval ( expression, interval );
Where expression is a function and timeout and interval are integers in milliseconds. setTimeout runs the timer once and runs the expression once whereas setInterval will run the expression every time the interval passes.
So in your case it would work something like this:
setInterval(function() {
//call $.ajax here
}, 5000); //5 seconds
As far as the Ajax goes, see jQuery's ajax() method. If you run an interval, there is nothing stopping you from calling the same ajax() from other places in your code.
If what you want is for an interval to run every 30 seconds until a user initiates a form submission...and then create a new interval after that, that is also possible:
setInterval() returns an integer which is the ID of the interval.
var id = setInterval(function() {
//call $.ajax here
}, 30000); // 30 seconds
If you store that ID in a variable, you can then call clearInterval(id) which will stop the progression.
Then you can reinstantiate the setInterval() call after you've completed your ajax form submission.