Current blockchain size is about 350GB and maximum size of prune block storage is only 99GB, so either 2GB or 99GB will take long time.
But AFAIK Bitcoin Core don't verify all block/transaction again, which safe some time.
I've just done a short test with my Bitcoin Core.
I synchronized the blockchain completely from the start and after it was done, pruned it. Unpruning it requires a reindex which we already know. I'm currently resynchronizing it and it appears that Bitcoin Core wipes the entire chainstate (reasonable since you have to run it with reindex flag) and retains the final X GB of blocks. This means that the validation will happen again and happens just as if you're synchronizing from the start, just that the final X GB is not redownloaded but still has to be built. AFAICT, it will actually take roughly the same time, no matter how much you prune.