Session lifetime limits for Workflow

HI all,
For one of our customers we have created custom UI based on Workflow Forms (Web).
When user opens this form it is opnening via URL like this: http://wf/LCM21/Form.aspx?s=cmlp4EnAfU27joXol3G3zQ
After a few minutes of inactivity (while user doesn't do anything in UI) user tries to refresh page with F5 and Workflow fails with error: "System.Exception: The dialog session named 'cmlp4EnAfU27joXol3G3zQ' was not found".
It looks like Workflow (or IIS?) has some session lifetime limitations, so how (and where?) could I change this limit?