Your best bet is to find an owner to rent from and avoid the fees completely. I don't think that there's a set standard from agents, but from what I've seen they will charge 1 months rent for 6 month contract, and beyond that they will charge 20% of the total arranged time. For less than six months they may charge a fraction of a months rent but this is where I've seen the most variability. For 2 year contracts (normally unfurnished, but who knows) 2 months commission is standard, plus 1 month damage deposit.
Aside from fees expect to put 1 month as a damage deposit and 1 month possible last month's rent (though this isn't asked for that frequently).