They need to be long enough that your shock is not the ultimate bump stop. Timbrens don't really count for extending bump stops - they move the initial contact point of the bumpstop lower in the suspension travel, but they are meant to compress a lot more, and don't change the ultimate bump stop point much. You need to extend the limit of your suspension travel.
If you don't have a direction from whoever is supplying the shock, remove your springs and fully articulate your axle and figure out where the shock hits full compression. Make bumpstop adjustments to make sure you never hit that full compression, and remember that rubber compresses, including bushings in the control arms, so leave yourself some safety margin.
Looks like you might have ARB KDSS skidplate in your profile pic - hopefully you have removed your KDSS, because it isn't going to work with LT shocks
