You should certainly be able to change the lease time.... but you don't often see more advanced settings exposed in home ADSL routers.
Sometimes there's a hidden advanced screen you can access by appending something to the router address in your browser's web bar.
I'll have a dig around re:Sagem, haven't seen many of those.
(By the way, just incase you weren't aware and were wondering, my reference to MAC address earlier was as in Media Access Control address... i.e. the unique identifier of your network card... rather than Mac as in the abbreviation of Macintosh).
Edit to add: Found a somewhat old (2004) Sagem manual which suggest its might be in Settings -> Network. Not sure if the menu structure has changed.