I don't think that a contract of retainership should have a clause like leaves, insurance etc should not be there. Of course, there can be reimbursement of certain expenses, like, travel in connection with office, fee paid to departments on behalf of the establishment etc.
In my opinion all employees cannot be retained as a retainer. Only those who can advise you on technical, managerial or other matters can be put as retainers. There is no meaning in engaging a worker or a junior supervisor by redesignating him as retainer. A retainer should not be following your office timing, leave rules, dress code etc.
There can be a retainership for more than 2 years, I think. What is important is the role.