Personally I doesn't like time limed demos which fell more crippled like just 30min...
Personal a nag (where you need to press a key on keyboard, not touch) in about 15 or 30 min would been better idea (and maybe a album limit). But it just a personal taste.
EDIT: About the nag, use a graphics nag, not a windows dialog, since these dialog can been easy passable by a autohotkey script. So there must been a some second wait time before it can been "disabled".