this indeed varies from box to box. The Echostar3000 for example can display 14 days' worth. Their new DVR7000 can display 4 weeks in the future as well as the previous two weeks. But this all relies on the provider transmitting the information in the first place. Fact is, most large providers in Europe deliberately send non-standard EPG info that only their own boxes/software can interpret - Sky and Canal+ being two prime culprits. Canal+NL even refuses to send the now/next info on their Mediaguard-encrypted channels, and shamefully mislead the public by saying it's not technically feasible (it is of course, and in fact the very same company in France manages it just fine...).
2old