(welcome!)
difficult to estimate, I think it mostly depends on the size of the venue (bigger venue = more tickets sold = more likely that fans will be queuing early I guess), if it's a weekday or a weekend and many other factors.
For smaller venues, when NIN played at Scala in 2013 there were around 5 people queuing at 9am and around 20-30 at 4pm. Most people arrived around 5pm-6pm if I remember correctly.