I've help several righhanders never a lefty to switch. It pretty much has taken two years. One year batting entirely on the leftside only and then another year learning to switch. The youngest was a 9 yr old. In each case though the player ended up prefering the left side. Mainly I think because through HS they just see so few LHP.
The HS players who have learned to switch are almost always one of the best hitters on their team Probably due to the dedication and extra practice. The older players who have learned or tried to learn SH has been entirely a mental makeup thing whether they could be devoted to it enough to make it work.
The college coaches I have spoke to about it like the Idea of keeping a player in the line up and being able to avoid a speciality pitcher for at least one guy in the line up. Those who didn't like a SH felt like they just never had a good one that really helped.