Sunset provision?

I am mulling over a provision that every bill passed by Congress should have an expiration date attached so the bill must be rethought in X years, ie a sunset provision.  If I add this, how would the number of years be specified?  Maybe Congress can make the decision on each bill but there is a maximum of 20 years?

Another provision I am thinking about is requiring that all bills passed by Congress must include the annual cost to the government and to the public for the life of the bill. This makes sense for budgeting and impact purposes. But what should be done if the included annual cost turns out not to be correct?