But I'm talking about putting a percentage % and the amount of MB so that we all know the percussion, the largest example could be the same as ESET NOD32.
In all honesty that figure would be no different than a slider, how they figure out how much and how far a scan has got is always going to be pretty much a best guess estimate.
Whatever is doing this has to know many things to be even close to accurate, total data on the disk/area being scanned, are the files compressed, what type of scan is being done, etc. etc.
You would get to know from previous scans 'roughly' how long this takes and you shouldn't stat a scan if you need to use the computer during this time. I never sit and watch progress of a scan, I'm getting on with other things.
Finally, running on-demand scans on a system with an on-access antivirus scanner is much depreciated as most of the scan content would be scanned if active or would have been dormant (until active). I rarely run on-demand scans, outside of in relation to forum topics.