When you don’t want to use Aura Of Decay Heal to stay in Reaper Form, taking Reaper Form definitely makes you want to hurry up between packs, because your only method of stay in it would be leeching health from enemy.
Also if you do this method, you want to minimize any other sources of health drain, such as acolyte and lich passives: “Blood Pact”, “Unclosing Wounds”, “Elixir Of Death” and “Mind Over Body”. Unless you also invest 10 points into “Volatile Blood”, which would mitigate the drain effect substantially.
Also using Health Potion between packs, when you cannot leech helps with staying in, because, as soo nas you are in combat Lich has so much leech, that Health Potions become kinds pointless for healing in combat.
Reaper Form also has the Node “Elixir Of Death” in it’s Skill Spec Tree, which doubles healing from health potions, while in it, this greatly helps with using health potiosn to stay in Reaper Form between Packs.
Since Reaper Form’s health decay does get increasingly stronger, you will not be able to sustain it indefinitely, but on a good build you can sustain it for 1-2 echoes.
Usually I try to reset Reaper Form every Echo.