Commission for rental renewal

Hi, I would like to ask fellow landlords if they need to continue paying the housing agents commission if the tenant renew the lease - after the first 2 years ? and how much commission would you pay ? I paid 1 month commission for the first 2 year lease and am thinking if i need to pay another one month commission if the tenant want to extend the lease by another 2 years ? What is after this renewal, the tenant wants to extend again ? The housing agent does not do much after the first TA (tenancy agreement) was signed - and I really find it hard to justify why i should keep paying her every renewal . She asked us to sign an agency agreement - during the first TA - but is this agency agreement - like the TA - need to be re-sign for every renewal ? Thanks