How much time depends on the manager and what he's trying to achieve. Let us say Roy had us playing 4-4-2 from day one, and wanted the senior squad (and therefore the reserve squad by necessity) to adopt it as plan A, then it would take at least 6 months for the players to find their feet in new roles, because everything changes for pretty much everyone. But he didn't do that.
So instead we must accept that the players will need only to learn a few minor things about differences in their roles from previous seasons, and since they are professionals and players with international experience, this should not take more than a few weeks of training every single day to master.
Furthermore, as he keeps reminding us, Roy has 35 years of managerial experience, so he has surely learned the most efficient manner of transferring his ideas and knowledge to the team during that time. Again, this should not take half a season of training every single day. What we see on the pitch week in and week out is what they are practicing each day at Melwood. Make up your own mind whether they're practicing the right stuff.
Since our standard of play went downhill at the start of the EPL season and hasn't improved in the months since, one can only say that Roy isn't likely to get us playing like champions during the next month and a bit to the transfer window. Pick the tactics to suit the team; get that wrong and heaven help you. Can't help the feeling Roy is more in need of divine intervention than time.