If you just wish to have your reports open all day along without a timeout , I am assuming you are using Excel Services for SharePoint 2010
In the central admin go to the - trusted datasource library > Session Management > Session Timeout and set this value to "-1". This should prevent your time out messages for ever.
But this is certainly not a good practice to add.
The better way is to have a session timeout set to 6 hours followed by the excel application pool in IIS to recycle once in every 24 hours (during non peak hours). Predicting performance of a service application in SharePoint is a highly non deterministic thing as there are galaxies of configurations,factors on different levels (os,iis,sql) that come into consideration.However as a general rule of thumb you can go with the above suggestion.