It depends on your storage device's performance, size of the snapshot to be commited, number of snapshots, and even ESX version (snapshot commits were improved in 4.0U2 I believe).
For making a guesstimate, you could always use a test VM, take a snapshot, transfer a large file (2GB+?) and then remove the snapshot and see how long it takes and time it.
It's somewhat faster to commit snapshots when the VM is powered off, since there aren't changes to the disk.