well, however long you want it I guess.. the things you have to think about is if you have a long looklength, the bait will act more naturally with regards to it not pinging back after 6 or 8 inches like with most hooklengths, but if the boilie is sitting next to the weight and a carp takes it, its got to move a whole foot or more (however long the hooklength) before it hooks itself, and any indication is sent to you that something is happening..
Personally, the chances of your bait landing almost ontop of your weight is quite good in my eyes.. theres no way, unless using a huge solid pva bag as apposed to mesh, that your bait i.e. area around the hook will weigh more than the 2 or 3oz weight.. so when it hits the water, the first thing to sink will be the weight, thus dragging the hook and pva bag down, directly above it.. so when it lands, itll land very very close to your weight.. some people drag the weight back a few inches, maybe to dislodge it from any silt it has gone in to, but it depends on the lake bed I guess..