One of the main principles of contemporary firewalls is protecting internal resources of the system from intruders and checking correctness of requests forwarding to the particular service . Restrictions on firewalls cause that in many situation potential client has limited availability of the service. Although current implementations of firewalls are able to determine permissible number of session per user, there is no mechanism which could adapt this value to expected traffic in the network. This paper presents a proposal approach based on forecasting and statistical methods that is able to determine permissible availability of the service for the particular user.