the longer you keep the snapshot the longer it takes to remove it.
Snapshots take the vmdk and make it read only and creates files that keep track of all the changes to the vm since the snapshot was taken.
Deleting a snapshot merges those files back into the vmdk.
I have had the GUI say deletion has timed out. But it is really still deleting . The bad thing is you can not edit any vm settings while it is deleting (i.e. disconnect network card.)
Your free space on you lun will also start to get used and if it fills up your VM will fail.
Through experience I have found you should delete snapshots as soon as you can.
