consolidating snapshots

Hi,

I have 2 VMDK files for about 4.5TB in my datastore.

I need to consolidate the disks.

1. how much space should be available in my datastore for the snapshot consolidation.

2. How long will it approximately take?

3. During this consolidation, will the users get affected? like server hung/slow performance.

The ESXI server is 5.5 and the vm is win2012R2.

Thanks,

M.Krishaan