There is simply no rule for that. It depends on the venue and its facilities, the band, the bus, the driver, the traffic when going there etc...

If the venue is close to where you live, go there a few days before the show and see where you think, the tour bus would find some parking space. Call the venue and see if a friendly person will help you out. Tell them you got a ticket and are coming from a long distance and you wanna make sure that the band is there before you go there yourself. Stuff like that.

I have never done anything like that myself, but this is what comes to my mind and makes sense...

at a concert where is the musicians tour bus usually parked at and how long do you have to wait. i am trying to get an autograph.