Don't change Swain's playstyle with the ult nerfs, it shan't work.
Another post said why balancing around CDR doesn't work. TD;LR, it doesn't work because it becomes mandatory like flash.
If you nerf Swain's ult to 30 seconds, it massively changes his play style. I think it will gut him, although it might be like Ryze's nerf, where people learn around it. No matter what though, it will massively change their play style. It also won't solve much. It will decrease his winrate. But it will just allow him to be OP every 30 seconds.
Whats the solution than Mr. Arakadia ?
Gate him more by mana. Currently it doesn't seem to be like he runs oom enouhg after he gets his mana items. This nerf plus the RoA nerfs will shred him alive I think. But if you gate him by mana, it won't change his playstyle and will stop him from going crazy with his ult.