A problem with re-setting cookies is that it can be annoying if you have your browser or other application set up to warn you about cookies.
In a script I wrote, I used the following technique:
Cookies were set with relatively short expiration dates, about 45 days. Each cookie's value included a timestamp representing the "set-time". Each time a cookie was read, if its set-time was more than 30 days ago, the cookie would be re-set.
I also reduced the number of cookies required by storing several values within each cookie. Most web applications don't bother doing this.