Summary
Forever takes place right after the Warcraft III Reforged: Forsaken Kingdom campaign and before Molten Core, in a 'reimagined original Azeroth'. Warcraft Wiki puts it in 25 ADP, the first year of World of Warcraft.
Confidence note
confirmed_official (after Forsaken Kingdom, before Molten Core); reported (25 ADP, per Warcraft Wiki only)