I know that feeling. I come here to learn too and help where I can.
I am an accidental DBA as well.
I would give them a high estimate as it is tricky to say with any certainty (as far as I am aware). It can rely on a lot of different things like database activity, physical server activity, available RAM/CPU, available tempdb space, disk I/O, etc.
Do you have a test instance of this database? If so, you could try running it against that and estimate based on the hardware differences. In my case, live runs about 3 times faster than test in most cases, but it depends on your systems.
Or you said you ran that command last week friday? How long did it take (roughly)? I believe the FULLSCAN option means it should take just as long to run it 2 times with a 5 minute wait inbetween or with a 5 week wait inbetween (presuming minimal data changes) so I'd expect it to be roughly the same time.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.