probably longer than that. They have no plans right now and keep adding 'classic' receivers. They don't even have an mpeg4 capable SD box in the pipeline or on the drawing boards, much less an inventory that they could swap out 15 million subs worth (and many people have more than one box, many have 4 or 5, so there's ALOT more than 15 million boxes out there)
Also, changing all the programming over to mpeg4 would require new encoders at the uplink center. Its more economical to switch HD over now since there's only a few channels and relatively few subscribers compared to the entire installed base. They only have to replace the encoders for existing HD channels and buy mpeg4 instead of mpeg2 encoders for new channels.
Besides, they have the bandwidth now to deliver all the programming they want/need. I don't see them adding much more SD content (atleast national, main package channels; they may add internationals, locals, etc). Most of the new content will be HD.