It varies...if you're talking bare custom chopper tanks, that is a LOT of work. It doesn't matter how much people think they are straight. 99% of the time they are not. Your tank is OEM, I'm guessing. Really you just need to hit it with epoxy primer after prepping and cleaning it. Then you can paint on top of that. Of course, that assumes it doesn't need bodywork.
Previously painted tanks, especially OEM, are not necessarily easier. For one thing if you're painting the whole tank you're going to want to fix scratches and any dents that might be on there. Even if it is fresh and you're trying to change the color, you have the decals to think about. 99% of the time they are under clearcoat. I have seen many tanks and other parts painted where the decals were masked off. Even if you're using the same color it looks like shit. Most people can't mask off precisely and even if you can there is going to be a noticeable difference most of the time. An "edge" under the clear that is visible to the eye. Most of the time...sometimes you get lucky. It's better to just strip that off and do it over especially if you're changing the color. Of course, this means you're going to go down to the metal most of the time and then you have to do some epoxy priming and oftentimes some bodywork to get it straightened out again since you basically sanded a valley into the paint to get the decal off. Then primer, then paint, etc. There are always variances...it's not a matter of shaking a can and bombing it.
Things get complicated really quick, don't they?