Changing slidingExpiration at runtime

[milop]

Hello.

I have a Forms authenticated site and, similar to what OWA provides, have a
"Public Computer" and "Private Computer" radio buttons.

For Public I want to set slidingExpiration to False and change the timeout.
For Private I want to set slidingExpiration to True and change the timeout.

How can I do this at runtime? The slidingExpiration property of the
FormsAuthentication class is ReadOnly.

Thanks in advance,

Mike