I was just wondering is there any average length of time from sending off your apllication form to hearing if you have got an interview? if so how long does it normally take?

there should be a point of contact such as an administrator named on the application procedure whom you have every right to contact and ask such questions without appearing pushy by asking the supervisor directly. The time period between applying nad hearing totally varies depending on the number of applicants - start date, who is around to interview etc.. but you have a right to know when you can expect to hear so you can carry on with other options - too often unsuccessful candidates are just left hanging..

Contact the person to whom you applied and ask politely when you can expect to hear from them.