Oracle 11g, Will droping partition reuse free space on bigfile tablespace
It will be available. Depending on your space management settings it can nearly fit the new partition entirely in the space or there might be some gaps, but the majority of space will be reused.

I would not expect that the grow stops immediately after 30 days depending on the fragmentations but it will eventually. Using uniform extends or at least a big minimal size for extends will help, especially if you expect large partitions.

When using LOBs retention might come into play as well.