Grenades with ubWeight 6-8 (stun, tear gas, mustard gas, mk2, smoke and flashbang) have a top range of about 20 tiles. All assuming a soldier with a 100 str. And this is just their "accurate" range. Beyond this max range, it's still possible to land a grenade but your CTH is all but nothing because not only are you being hit with a huge cth penalty, you're also getting your final cth cut in half.

I tested a throw using Gus [STR 82, breath 83, crouching, throwing mk2 grenade) and he ends up with a 12 "maxRange". Although the red crosshair only went out to 9 tiles and I'm not sure why that happens.