It is unknow but is a very small chance
As you play through the demo multiple times, there is a small chance that you will be able to activate the secret mission. This mission has you go to an island where there are no wild Pokémon. On this island, you will encounter Team Aqua or Team Magma and have to team up with May. The team you face is random. - Serebii
This video show a guy that Soft Rest to find a Special Mission and after 253 Soft Resets he got a Special Mission.
This talks about a ''rare chance'' to find it along with other sites.
It is really unknow and only Game Freak, ORAS demo developers know what chance it is to find, but if you see from the video, 252 SR can mean a possible rare chance, maybe less than 1%.. so that is normal that your SR is taking a while.
Hope this helps (a little).