Lets's put it this way, if you have a fast computer and you don't want to release your addon to the community, than for the Pilot View and first LOD (more or less 0.350 distance) you can use as much polys as you want,
ofcourse not exagerating but let's say that for a weapon you can go until let's say 4.000 polys but only if you have a fast computer and a good Graphics Card, and than as you make the distance LODS you start decreasing polys.
BIS weapons are more less like this:
Pilot View 600 to 800 Polys
LOD 0.350 450 to 550 Polys
LOD 1.000 250 Polys
LOD 1500 120 to 200 Polys
LOD 2000 50 to 100 Polys.
LOD 3500 4 Polys.
As you may have guessed as the distance inceases you don't need to have much detail cause you won't see the weapon very well so decreasing the polys is ofcourse a must.
If your idea is to make for example a weapon for you and for the community, than my sugestion is to don't make the base model with more than 2.000 polys, and also don't use big texture files.
For a soldier the polys used by BIS if I remenber (NOT SURE HERE), are more or less 3.500 Polys for the base model.
So if you wanna creat a soldier achieving some quality but not
over-exageratoing than I would say go until 4.000 too, but make the distance LODs well or a lot of people will say that your model Lags the game.
I think you have an idea now, Good Luck mate