The tribunal aims to hear most cases within six months of the ET1 being lodged. Obviously short, simple claims (e.g. unlawful deduction from wages cases) can sometimes be slotted in earlier than multi-day complex claims (e.g. equal pay cases).
You will be required to tell the tribunal how many witnesses you intend to call and how long you think the hearing will last. If there are any dates you would find hard to make (e.g. due to having booked a holiday) you should notify the tribunal early on. It is important to think about this carefully because it is likely to be very difficult to change arrangements, e.g. the length of a hearing, once it has been set and listed in the tribunal calendar.